What are the chances I'm pregnant.?

Me and hubby and aren't actively ttc yet (not really monitoring temps, ovulation etc.), more just not being careful and waiting to see what happens. My cycles used to be quite irregular but for the last year or so they have been 33-35 days apart. My last period was on 10th Jan, and I've had nothing since except light spotting for a day on the 18th Feb, which definately wasn't my period, but seems slightly late for implantation bleeding? Anyway to cut a long story short my period is now 2 weeks late, I have pain in my lower back, really dodgy tummy the last few days, little flutters of nausea that come and go, and constant heartburn. I've taken countless hpts, from a week before my period was due right up to last week and got all bfns, except one that looked like it might have had a very very very faint line, but that could have been evaporation and my hubby didn't seem to think it looked like a line. I guess I just want to know what the chances I could be pregnant are; I have all the symptoms, but not bfp!