Which Painkillers Can Be Taken In Urticaria?

I am 16, and suffer from chronic solar, heat and pressure urticaria; as well as vesibulitus and interstitial cystitis. I take antihystemines and have recently been put on a 10mg daily dose of Nortriptyline. I hvae been told that due to my Urticaria the only pain killers i can take is Pracetamol which do not help at all and still leave my in extreme pain and discomfort. Why would other painkillers make it worse? And do you know of others that i can take?