[Always *]

I know you do not believe you're pregnant, but accidents can happen even with birth control, so given that it's been about 5 weeks since you're last period and this unusual dark blood symptom, along with the fatigue and bloating, I would like to recommend that you rule out pregnancy any how. There is a such thing as implantation bleeding (there is probably a better medical term, but this is the term I have) where sometimes you might bleed a little even though you're pregnant during the first few weeks of pregnancy - this caused my friend to think she wasn't pregnant when she was 17 because she'd always used condoms, only then her next period didn't come. I brought it up to a doctor, and it turns out that that's not an ultra rare phenomenon... It's not super common either but I'm just letting you know that it happens so that you'll consider ruling it out.

Otherwise, there could be a huge number of factors that are affecting this; high levels of stress or illness (e.g. the flu), large amounts of weight loss, etc. could have caused you to essentially miss you period and now your body is just eliminating the small bits that did get through a little late. That said, I am just making a non-doctor/nurse/health care professional educated guess, if you're genuinely concerned, you may want to consider following up with a doctor just to get things checked out and make sure everything is in order, especially if you've ruled out pregnancy, because there is an endless list of things that could be causing it, ranging from something reasonably "minor" like stress and minor illnesses, to something more serious like an STI or PCOS. It could also seriously just be a one time thing that'll never happen again because you ate, like, pie or something (seriously, it could literally be because of a random one off circumstance).