Upset stomach, possibly constipation after tonsillectomy?

Hi. To make a long story short, I got my tonsils out 3 weeks ago. I was taking liquid percaset with codeine and some antibiotic. Since then I havent really been able to go to the bathroom regularly. (Now i MIGHT go once a day and it's only like a little pebble) Regardless, since then, I have had a horrible upset stomach, which starts in the morning and pretty much lasts all day, getting worse after I eat. It's the top of my stomach that hurts and it looks bloated. Could all of these problems be from the medicine? Can anyone help? I can't understand why I feel like this or what to do? Also, I have post nasal drip and almost throw up in the morning gagging it all up, maybe thats why my belly hurts. (I've had , but on the pill, didnt ejac. inside me, so not really worried abour preg b/c I felt like this before hand) PLEASE HELP