I had my last period on 3april 2011 and took pregnancy test on 7th may 2011 which turned out to be negative also I had no pregnanacy syptoms but to the surprise I started bleeding on morning of 8th may 2011 with stomach cramps and lower back ache is it implantation bleeding or menses

It sounds more like a period. Implantation bleeding/spotting is not noticed by most, and it usually happens sooner in a cycle. Is the bleeding similar to that of a regular period?

Since you had a negative result on the test I'd say it's not implantation. Not to mention implantation is lighter than a period flow it's usually a pinkish or brownish spotting that only lasts a day or so and many women don't even notice implantation bleeding if they even have it. The cramps and back pain can come with menstruation. I'd say you're not. Only way to be sure is wait for a missed period and take another test.