I had unprotected sex on the 11th of this month and my period came on the 14th which is early because it usually comes on the 19th, I bleed very light for the 1st day thereafter I bleed heavy and by the 18th my period was done, then on the 20th and had a pinkish brown discharge on my underwear and when I wiped there was more.. when I got home form work and when to the loo there was more discharge when I wiped in the same pinkish brown discharge... now it's a week later and I notice that my breast are swollen and I feel really tired at work and for the past 3days I am very constipated.. I have constant headaches... please advise me the possibility that I could be pregnant

Any time you have penile insertion there is the chance of pregnancy. We can't tell, only a pregnancy test or physician exam can tell what the issue is.