YOU SAID:

I cannot come to your party as I have an upset stomach

INTO JAPANESE

お腹の調子が悪いのでパーティーに参加できません

BACK INTO ENGLISH

I can't join the party because I'm not feeling well

INTO JAPANESE

体調が悪いのでパーティーに参加できません

BACK INTO ENGLISH

I can't join the party because I'm sick

INTO JAPANESE

病気なのでパーティーに参加できません

BACK INTO ENGLISH

I can't join the party because I'm sick