"What is wrong with me?"
Translation:Beth sy'n bod arna i?!
Illness or a general state of being are considered to be 'on' someone in Welsh, and also the other Celtic languages.
Thus:- Mae annwyd arna i = I have a cold (lit:- there is a cold on me)
Mae ofn arna i = I am afraid (lit:- There is fear on me)
Mae syched arna i = I am thirsty (lit:- There is thirst on me)
Your suggestion is not a natural Welsh sentence but a literal translation from English.