Could I Be Pregnant Despite Regular Periods and Negative Tests?

I want complete clarity on this . I had an intercourse on oct 24 which includes no vaginal penetration no ejaculation. it only includes anal penetration and rubbing around anal region for about 10 sec. there were bleedings on Nov 8th dec 6th Jan 3rd and Feb 2nd .These bleeding seemed to be regular flow and last for 3 to 5 days . also did a urine pregnancy test on Dec 30 with evening urine (might be diluted) showed clear negative. later on Jan 29 went to a clinic and did ultrasound scan and they said no pregnancy. Is there any sort of possibility of pregnancy in this . because i have seen cases on Google where a women had regular periods and delivered a baby unknowingly about her pregnancy until labour. my question is even some cases exist where a scan can't find pregnancy. is that possible at all at time after 13 weeks of intercourse. can I beleive the results for sure or do I need any further tests. Please give clarity on this. I have been thinking a lot from 3 months . I need peace.