Light period/possible pregnancy??

Me and my husbands only form of birth control is the pill. It has worked for the last few months we’ve been married and few years we’ve been together. Today is thursday and I start my period on the same tuesday every month and it ends on the following thursday (so only 3 days) and its usually the same thing every month...not light but not heavy, just in between. Its normal. So i started something this past tuesday, its kind of bleedy but its more dark and definitely super light. Like not even anything. The last few weeks i have had acid reflux and i never have that. Ive also had cramps in the bottom of my stomach and the bottom of my vagina. I took a pregnancy test today and it said not pregnant. I just dont know why my period is so off this month, could there be something else going on inside my body?