Am I pregnant or just plain paranoid?

Ok I think I might be being a tad paranoid, but just wanted to seek some more advice on this. I have been on the pill (microgynon 30) for about a year now and up until now had no issues. BUT, for 2 weeks around christmas I was on pretty strong antibiotics for a throat infection (I can't remember the name of them). Neither my GP nor pharmacist advised me against taking any other action to protect myself while I was on them other than taking the pill and as such I didn't. A week before I was due to come off the set of pills I was on at the time I had a really heavy period - only lasted three days which is unusual for me. After a bit of research online I found that breakthrough bleeding isn't uncommon and thought nothing of it - until now. I'm now about 3days late for my next period. I've spent the last week feeling really nauseous off and on (worse in mornings), I have occasional stomach pains - not too dissimilar to period pains - and my are REALLY sore! After a lot of panicking I took a pregnancy test this morning. It did come back negative but is there any chance I am actually pregnant? I know I'm probably overreacting but any advise here would be great!