spotting after my periods

hi,my period date usually shifts some days ahead of before..my last date for my period was 22 april.. i had sex with my boyfriend on the 17th of may with protection.. i took a pregnancy test on 18th which said negative..only that the preg test said 'to be taken on the day of missed period'.. which i ignored n took the test anyway.. i got my period on 19th aftrnoon and had normal bleeding by the second day.. it lasted til 23rd may.. n on 28th of may i experienced spotting which lasted fr vry less time.. it was muddy red in colour.. then again i experienced spotting on 30th lasting for just a few mins.. i am very confused as this has happned for the 1st time.. could i be pregnant? waiting for your reply