What Causes Yellowish Discharge With Cramps After Period?

hi, i have my period about a week go, by thursday it ended but i started to have white yellowish discharge with cramps- , I had sex on that saturday, sunday came and i had really bad cramping along with lower back pain on my right side. I m currrently on Birth control, I forgot to take one yesterday, I took it a few hours ago, is this serious? Also I was told to skip a period by using birth control to help stop me from bleeding.