Std? Yeast infection? HELP

Okay so last weekend my husband and I had a getaway and we had sex about 15 times in the 48hrs we were gone and he came in me 13 of the 15 times. I am almost 6 months PP and on the pill. When we got home I felt really sore and swollen down there. I thought that would be expected so i let it be. Then it started to itch SOOO BAD and slightly burn and i had some discharge that was thicker then normal and had a yellowish tint. So i thought i had a yeast infection so i used monistat and it was like FIRE in my vagina. So i got it out and made an apt for my gyno on Monday (i called them on a Friday so i had to wait the weekend.) But come Monday im back to normal??? No discharge, burn, itch..nothing!!!!! Just gone. This happens a lot actually. Anytime my hubby and i fuck a lot and i get a lot of cum this happens. Let me say this!!! I am married and Both of us are std fee (we have been checked recently) why does this happen???