Confused about tests.

If you have irregular periods, how do you know when your supposed to test and it be an accurate result? I've always been so confused about this. I have endo and am on the pill however have had a couple of slip ups with it recently due to things being so hectic in my personal life, so how do I determine which is the best time to test? I have an appointment with my GP on Wednesday, and did a home test last Friday which I think I saw a faint line but then was warned that the brand I was using wasn't the best, so was waiting for my appointment but how will I know if it's too soon? The doctor was the one who suggested I could be pregnant on the phone as I've been having super weird symptoms but I don't know I'm really confused. Last had sex on 02/08 so would enough time had passed? Had less than half a days bleeding a couple of days after but thought nothing of it until now. Ive been having like a FWB kind of thing going on with this guy for a while now and I really dread telling him if I am. Also had an internal scan about a week or so after sleeping with him, however the lady couldn't tell me the result (am currently waiting for those but it's with a private hospital so I don't know if my GP will have those results) any one got any ideas? Sorry for the long post, I just had to get it off my chest 😮 x