I've had something very similar.
I had what I think was my last period 23/09-29/09... It was a few days early and much lighter than normal with no pain or other pms symptoms, hence "what I think was" my period. Had unprotected sex for over a year post the expiry of my implant with nothing similar happening.
Then a week after, on the 09/10, I woke up with a huge headache, and then 2 hours later I got some almost clear stringy discharge and had brown spotting, which later turned to huge clots but not a lot of blood otherwise. I also started going really lightheaded. Saw my doc the next day, took a urine pregnancy test at home before which was negative, and my doc did one too, again negative. Bleeding didn't get any worse and stopped on Monday, and I had no pain at all during the bleeding time.
Since it all started on the 9th I've been nauseous every day for most of the morning. A little lightheaded on and off and now getting slight discomfort in my lower abdomen, some serious bloating and sensitive boobs (which have also felt heavier the last 2-3 weeks).
Anyway, my docs referred and fastracked me to colposcopy which I have tomorrow morning, and for an ultrasound to look into the bleeding.
Today I did a test, left it at 5 mins negative, came back almost 2 hours later and there's a distinct positive line... Doesn't look at all like an evap line, its completely solid. Kicking myself for not checking it at the 10 minute mark as now I'm confused, gonna do another in the morning to be sure of what the outcome is.
(last time I had unportected sex was around my ovulation time in September 12th and before)
Hope everything turns out okay for you OP, I'd definitely get to your docs, mine seems to be taking it very seriously, even told me to go to A&E if I got even a little worse.