All right so my girlfriend and I had have, always using a condom and her period was towards the first weekend of April. During then we thought some semen may have leaked from the condom and just to put worries to rest she took plan-b the next day. Now this week, she's been complaining about being more tired than normal and also not being able to fit into her normal jeans and shorts because of her thighs and waist, so I'm not sure if its just normal weight gain, the plan-b pill, her coming period, or that she is pregnant?

You had sex... no birth control is 100% effective even if you use them correctly.

Is she pregnant?. you have to wait until she is at least two weeks late and have her do a Home pregnancy test when she wakes up in the morning the first time she pees that day to find out... otherwise all you are doing is making a wild guess..