The exact same thing happened to me back in January. I experienced almost every possible pregnancy symptom. At 14DPO, I got a feint line within about 3 minutes which DH could see so I made an appointment at the doctor but the BT came back negative. All subsequent HPT's came back negative too. Luckily I took the stick to the doctor and he confirmed that yes, he would have read it as a positive too. AF turned up two days later.

The doctors explanation was that fertilisation took place and may have temporarily implanted (causing an initial squirt of HcG hormone) but didnt "stick" long enough for a full amount of hormone to be released. He said it isnt classed as a miscarriage but classed as a chemical pregnancy. He also said (and I have since seen it on the web) that 50% of all women experience chemical pregnancies and they dont even know it.