Got flu. Was prescribed tamiflu (oseltamivir). Then after a day partied and drank. Now little better but nausea, feeling and pain in right lower stomach. It serious?

Probably not. The flu can be a very serious illness, and deciding to drink one day after starting to recover from it probably wasn't your best move. Having said that, however, you will most likely continue to improve. You should drink lots of (nonalcoholic) liquids, and try to rest for the next few days. If your symptoms persist longer than a week, then you probably should be seen by your doctor.