There's always a chance of infection creeping in to the root no matter how good or careful the dentist is. In fact, it can even happen in the absence of any treatment. You really do need to get back to the dentist and get it sorted, even though you'd rather not. Meantime, keep taking the painkillers and, if you don't have to drive or whatever, and if you're OK with it, take a shot of whisky or brandy to help you sleep. I find baby teething gel helps too.
Like you, I thought I'd done with mine just before Xmas, but have had to go back to the dentist again recently because of one huge abcess for which I'm still taking antibiotics. Sadly, it happens.