There are so many possible variables, it is next to impossible to tell you specifically what is wrong in a forum like this. However, I am highly suspicious of two things:
1) You said you tested the safelight, but you didn't tell us how or the results.
2) You have fogged your paper. Sorry, but you have. Developing a sheet without further exposure may show only a "little" fogging, but it is possible that you have "flashed" the paper, so that any additional exposure plus the fogging may push it over the threshold and it goes darker. (edit)
Buy new paper and do a proper safelight test
or: http://www.ilfordphoto.com/Webfiles/...7111757603.pdf (page 2)
and get back to us.