Pregnant or not pregnant?

My girlfriend has been on the depo shot the past 2 years but recently she switched to loestrin 24 about a month ago. We always wear a condom, but the only time we had im worried about is when i finished the condom slipped off when i pulled out (she was on top) and this was almost a month ago. she got her period last month and she has her period now but its a little early? she has missed a few pills sometimes, maybe this is why? she has been having a lot of gas like farting lately, but no constipation and she had diarrhea for 4 days straight last week. of what i told you, what is your opinion. not pregnant? from what she says she hasnt been getting any other symptoms like feeling sick or random frequent cramps, tiredness, or weird cravings and eating a lot. idk i get to thinking to much.