My neck hurts because I was reading with my head down.
「首が痛い」は my neck hurts. もしくは my neck is in painと言うこともできますが前者ほうが自然な言い方だと思います。
「下を見ながら読書」は "read with my head down"ですので、それを「読書していた」ということですので現在進行形の過去形にしてあります。
When I read with my head down, it makes my neck hurt.
My neck is hurting. It must have been from reading with my head down.
Do you think my neck hurts because I was reading with my head down?
When I read with my head down, it makes my neck hurt.
My neck is hurting. It must have been from reading with my head down.
Do you think my neck hurts because I was reading with my head down?
Would reading with my head down cause pain in my neck?
I am feeling pain in my neck from reading with my head down.
Reading with my head down causes my neck to hurt.
When I read with my head down, it makes my neck hurt.（下を向いて本を読むと首が痛くなります）
My neck is hurting. It must have been from reading with my head down.（首が痛いです。下を向いて本を読んだからだと思います）
Do you think my neck hurts because I was reading with my head down?（下を向いて本を読んだから首が痛いということですか）
Would reading with my head down cause pain in my neck?（下を向いて本を読むと首が痛くなりますか）
I am feeling pain in my neck from reading with my head down.（下を向いて本を読んだので首が痛いです）
Reading with my head down causes my neck to hurt.（下を向いて本を読むと首が痛くなります）
I use 'I have neck ache' to show that I am feeling the pain in my neck at this current point in time. Using the words 'from looking down while reading' explain how I got neck ache from reading and looking down.
I have neck ache' は今現在首に痛みがあることを表します。
'from looking down while reading'（下を向いて読んでいたので）は、どのようにして首が痛くなったかを説明します。
My neck got stiff because of reading for an extended period of time in one position.
Native speakers refer to the pain in one's neck (typically a result of stretching the neck muscles in one direction for a long time) as "stiffness" or they say something like "I have a stiff neck" or "My neck is stiff." Such stiffness usually includes the inability of moving the neck because of it hurting when one tries to do so.
Using the auxiliary verb "to get" is a common way of expressing this issue in colloquial English. Although it is not required to explain the cause of one's stiffness in casual conversation, such an explanation can be provided by using the word "because . . ." or "from. . . ." and then stating the reason.
ネイティブスピーカーは（首の筋肉を長時間同じ方向に伸ばし続けたことによる）首の痛みを "stiffness"（筋肉の凝り）といいます。
あるいは、
"I have a stiff neck"（首が凝っている）
または
"My neck is stiff"（首が凝っている）
のように言います。
こうした "stiffness" は普通、痛みにより首が動かせないときをいいます。
口語でこれについて言うときには、助動詞の "to get" がよく使われます。
"stiffness" の原因はカジュアルな会話では説明する必要ありませんが、もし説明するなら、"because..."（...が理由で）や "from..."（...が理由で）が使えます。
My neck is hurting because I bent it for too long while reading my book.
My neck hurts because I bent my head down for too long while looking at my phone.
English speakers will say that their neck hurts if they are constantly looking down at their phone or a book because after awhile, your neck gets sore when you don't move it around at all and keep it in the same position.
My neck is in pain because i was reading with my head down for a long time.
My neck is in pain from looking up every time i speak to you.
My neck is in pain because i was reading with my head down for a long time. You can say this to express that your neck is in pain because of the posture you in whilst reading. This will allow the listener to understand if you look like you are in pain.
My neck is in pain from looking up every time i speak to you. - this expresses your neck is in pain and why, the reason for the pain is because your head is facing upwards when you are speaking to someone who is very tall.
My neck is in pain because I was reading with my head down for a long time.（長時間下を向いて読んでいたから首が痛い）
本を読んでいたときの姿勢のせいで首が痛くなったことを表します。周りから見て痛そうだなと分かるくらいなら、このように言えばその理由を伝えることができます。
My neck is in pain from looking up every time I speak to you.（あなたと話す度に頭を上げていたから首が痛い）
これは首が痛いこととその理由を伝えています。痛みの理由は、相手が身長の高い人で話す度に頭を上げていたことです。
My neck hurts because I was reading with my head down
I was reading with my neck down and now my neck hurts
When you want to explain to your friend that your neck hurts because you were reading with your neck down; then you may explain this in the following ways:
-My neck hurts because I was reading with my head down
-I was reading with my neck down and now my neck hurts
「下を向いて読んでいたので首が痛い」は、次のように言えます。
-My neck hurts because I was reading with my head down（下を向いて読んでいたので首が痛い）
-I was reading with my neck down and now my neck hurt（下を向いて読んでいたので首が痛い）
My neck hurts because I was reading with my head down.
こんにちは。
様々な言い方ができると思いますが、例えば下記のような表現はいかがでしょうか：
・My neck hurts because I was reading with my head down.
下を見ながら読書していたので首が痛いです。
シンプルに上記のように言うことができます。
my neck hurts で「首が痛い」となります。
ぜひ参考にしてください。
アメリカ合衆国 
日本 
イギリス 
南アフリカ 
