Two light periods in a row, negative tests?

I had unprotected vaginal sex on october 20th. he did not cum in me or anything, and weve done stuff like this multiple times n ive never gotten pregnant from it. and as far as i know he wasnt close to cumming. the next day we had unprotected anal sex, and he did cum in me. my periods r fairly irregular, but after a pregnancy scare earlier last year that started happening during the first week of the month. my last normal period was october 9th. after we had unprotected sex those times, i havnt had any sex since. on november 14th, i had a "period". but it was lighter than it normally is and it lasted five days. it still was enough to soak up in tampons, but it wasnt nearly as much as i normally get. i took a pregnancy test around december 14th and it came out negative. i know it should b accurate by then, and plus it was a first response test. on december 18th i got another period. again it wasnt my normal bleeding. it was still lighter than my normal period, but it soaked up more of the tampon then my previous period did at times. again it lasted exactly five days again. normally my period is sorta medium flow that lasts like 3 or 4 days and tapers off the rest of the week. today is january 7th, and just yesterday i took another pregnancy test, and it came out negative as well. i have no pregnancy symptoms, and im testing negative when its spaced out far enough to where they should b reading accuratly by now. but i cant seem to figure out why my periods have become so light all of a sudden the month after i had sex, and then stay that way the month after as well? any help or advice please?