can this even happen, this makes no sense to me

Emma • 24, mum to a 3 year old boy, DV survivor.
Hi guys I've been on the pill for a few months now, and have only used this as my safety net. However I've not really left myself lately and took a test and there is a very faint positive line there... has anyone fell pregnant on the pill?
I had sex on the 18th June and I had a period 22nd-29th June. Took the test on the 1st July, it just doesn't make sense to me 🙈 I'm a mum already. Would love another baby but these circumstances don't make sense to me!!