I had a 1day heavy period &3spotting days. can that be a result of the plan b pill ? should i be worried. i took it an hour after unprotected sex

Likely okay: The fact that you took the Plan B 1 hour after unprotected sex is about as good of prevention of pregnancy as possible using that method. Your period definitely sounds within normal. 85% of pregnancies when Plan B taken right away are prevented. You can always take home pregnancy test, but I bet it will be negative.

Irregular period: I am assuming the bleeding is at the time of your period and not right after Plan B. It may be but would still do blood/urine HCG as Plan B is only 75-95% effective depending on type used. It is less effective in women weighing > 165 pounds. If negaitve would consider sonogram to look for ectopic position/retained products