腰痛って英語でなんて言うの？

Well, usually back or lower back - precisely where it is, is not important - unless you are describing your symptoms to a doctor.
To give one gyp = to give someone pain or trouble.
"My toothache is really giving me gyp!" (informal)

You can use any of these examples, 'backache' would be the most common word. You can say something like, "I have lower backache." This explains where the pain is.

My back is really hurting.
Meaning= really hurting means a lot of pain.

My back pain is really bad today.
MEANING= really bad = hurting a lot, lots of pain

I have a lot of pain in my lower back
MEANING= you could say this for any part in your body instead of lower back

You could also say..
ouch, my back is all seized up today
My back is super painful today
My back aches all over
My back cracks a lot at the moment which is painful.

Hope this helps! ^ ^

Backache is pain that affects one's back. If the pain is concentrated in one's lower back, it is called lumbago. Lumbago can be very painful and make it extremely difficult for a person to stand up from a sitting position. If you have lumbago, it's better for you to either see a doctor or take pain killers.
So, you may say to the doctor:
I have got lumbago and it's very painful.

Lower back pain: is specific to the part of your back that is the lower part but when you say backache you are referring to your whole back.
Phrases like "My back is killing me!" also indicate back pain. Where "killing me" is a figurative speech that is commonly used in American culture to suggest something extreme.

Hope this helps!