Side effects from weight loss on birth control?

I'm on Yasmin and am underweight. I've lost about 5 pounds in the last two weeks and experienced some side effects such as acne and cramps, and on Wednesday I actually started a light period which is still going on at the same rate even though my period isn't supposed to start until Tuesday. I'm still taking the pills and am seeing my boyfriend on Monday. So, I was wondering if eating more over the next two days would stop this. Thanks.