Hello,
Sorry you're having a worrying time. I don't know if hayfever pills can mess up your cycle, though the fact you've just moved to London made me wonder if you've had the stress of moving recently and if that may have had an effect? But obviously the best thing is to seek a medical opinion, you can still call NHS Direct on 0845 46 47 even if you're between Doctors, I've found them to be really helpful over the phone.
Hope you don't have to wait too long for the positive result, good luck!
T