What Causes Sickness, Nausea And Lack Of Appetite After Having Protected Sex?

I am a 20 female i have been having sex but i am on a pill and we use a comdom everytime!!! we even check the condom afterward just incase and they have always appeared to be fine. The lst time we had sex was around christmas eve. but last thursday vening I began to feel sick. I thought mabey it was just the stomach bug but scince then usually i will feel fine all day till around 4 ish then I feel nauseous? I have not actually gotten sick but just feel nasty. I have no appetite at night anymore and havent eaten dinner for atleast a week now. I have not officially mssed a period should be due for one this week? but cant expain these symptoms!! I have been diagnosed with cysts on my ovries that rupture and also a stomach ulcer. would these be acting up mabey causeing me to be sick? Sometimes I just feel that it is the stomach bug but it wouldnt last this long would it? I dont want to even think this but is it possible I could be pregnant?