Yep, entirely possible to concieve first go - yes, I know from experience. I get crampy before the BFP, kinda like AF is stuck.
You can be lucky first go but it may still be too early for a HPT to work. Some women do not secrete enough HCG in their urine until 6 weeks - ie 2 weeks after AF is due.
Test in a couple of days time - if it is still BFN and you are usually regular, then I'd suggest go and see your Dr.
Good luck. Hope it is a BFP for you - it just might take a little while to show so be patient. Your symptoms do sound promising.
GIDGET