具合がわるいので先に行ってもいいですか？って英語でなんて言うの？

"I'm feeling like I'm going to be sick/vomit please can I go ahead of you." this explains that you feel like you are going to throw up/vomit/get sick so asks if you can go in front of them.

"I feel really ill please can I go first." this explains that you feel very ill/sick and asks if you can go in before them.

The act of going before someone in a queue or placing yourself at the front of the queue is referred to as 'jumping the queue' if you go to the front - or, 'jumping in' if you place yourself between two people ahead of you in the queue. Obviously in this case, it would e preferrable to 'jump queue' due to the urgency of this predicament:
"Excuse me, I'm really unwell. May I jump the queue?

The best way to express this idea is to ask a question and explain why you are asking it. This is best done with two sentences.

When you want to get ahead of someone in line you can say:

Can I jump in front of you?
Can I get in front of you?
Can I get ahead of you?

All of these ask the same thing in a friendly, natural way.

If you are going to vomit, you can say:

going to be sick
going to throw up
going to vomit

In most cases, someone will allow you to go first.

*May I please go before you? I may throw up anytime.
This is a way to show how urgent you need to use the bathroom.
For example:
A:May I please go before you? I may throw up anytime.
B:Yes that is fine.

Some times, when we feel unwell... we really cannot wait in line for the bathroom!
It is polite to explain the reason you feel you should be allowed to "Jump the
queue.." or "skip the line...and " I think I am gonna vomit"...usually does the trick!

When you want to ask a person at the front of the line to go in before them, then you may ask in the following ways:
-May I please go in before you? I fee like I'm about to throw up
-May I please go in before you because I'm not feeling well