The pain in the ass you're experiencing is probably from the Benzyl Alcohol (BA) and/or Benzyl Benzoate (BB) that is used to dissolve the tren. That stuff stings like sh1t. Next time try pinning it in your upper thigh (above balls level) and this should minimize the p.i.p. (post injection pain). Definitely don't throw it out yet as it might be good. Tren's a lot rougher than test, but it's potent sh1t. Warm the vial or even just the syringe under hot water for a few minutes. Inject slowly.
Give it a week or two to verify it's legitimacy while monitoring your sides. One of the most common is intense night sweats. If you wake up soaking wet one morning, you'll know why.