BFP and Brown spotting

Hello ladies !
Excited news, got my BFP this morning ! I cannot believe it as I was told I did not ovulate and was supposed to start <a href="">IUI</a> in a month or so.
Pregnancy confirmed by blood test (4 to 5 weeks) but I noticed some light watery brown spotting this afternoon.
Anything to worry about ?
Many thanks for your help...