How likely is it for me to miss my period but not be pregnant after having consistent/regular cycles for years?

I was on birth control from early 2015 until August 2025, fully going off of it in September 2025, and during that time I have had fairly regular periods, beginning on or before my expected start date.

I've been NTNP since January, and have gotten my period on time or even slightly early every month. However, as of today, I am 4 days late, which is pretty unusual for me. What's confusing me a bit is that I've taken 3 tests (one a week ago, one 4 days ago, and one yesterday) that have been definitively negative.

I thought I was getting my period today because last night I had some browner discharge/spotting, and then this morning had a small amount of blood on my toilet paper but otherwise nothing more than the brown spotting all day (defining as such because I haven't had enough to require the use of a period product).

I think it's likely that I ovulated later than expected, but I also was sexually active a couple times following my expected ovulation date.

I am new to the TTC community and not many of my friends have gone through pregnancy, so I'm not sure if it's just a coincidence and I shouldn't get my hopes up, or if my hcg isn't strong enough right now to show up on a test. It's just really throwing me off because I have never been this late EVER but now I'm getting these bleeding-adjacent symptoms.