what is causing change in menstrual period?

I have a question about my period. It has been quite odd lately. On March 26 I began and that cycle ended on March 29. My period was the normal amount, but only lasting 3 days when it normally was 5 to 6. Since then I have gotten my period on time every month, expect for the length has been the same 3 to 4 days. Although the amount of blood is seemly normal for the first two days, it would then dwindle off, something that has never happened before. I did have sex March 21, with a condom, but I haven't since then. At first I though I was pregnant, but since I'm unable to get my hands on a test I sat it out till now. If I am I would be three months along, however I show no signs. Plus I believe by now I would have a lump or something, and I have no such thing (I am able to suck in my stomach to the point of being able to almost touch my spine, so I would notice something) I haven't changed my diet, no stress, nothing changed expect my period. Any ideas?