I think I may be 18 weeks pregnant, but I had what seemed like a period for 3 months, the 1st two months I started on time, but the periods were only 2.5 or 1.5 days long. Then the 3nd one was late by 3 days and only lasted 1.5 days, now its time for the 4th one and it is 6 days late. I took digtal pregnancy test in the 1st and 2nd month and they all came out negative, I haven't taken anymore. Has anyone ever experinced anything like this?

Go to your doctor for a blood test and further exam. It can be anything from infections, to STD, many other illness and of course pregnancy.