Could I still be pregnant although the last time i had intercourse was 25 days ago on the very last day of my period and I took a pregnancy test a week ago and it was negative?

No, you are not pregnant. Around the 14th day from the first day of your period is when you ovulate. That mean the ovaries releases the egg and it becomes embedded in the uterus wall. From that point, you have a good chance of getting pregnant if you have sex. You also have to remember, everybody is different. The majority of females won't get pregnant in this situation but you never know what will happen. The best advise I have is for you to get on some sort of Birth Control. Your boyfriend also needs to wear a rubber.