Yes they appeared quite quick big red itchy lumps behind my left knee almost within a few minutes and quite large only on one knee?

What happened first?: What happened before the lumps appeared? Were you stung/bitten by an insect? Had you rubbed against some plants/weeds? We need more information to provide the best answer. Possibly using topical Benadryl (diphenhydramine) or taking zyrtec/benadryl/claritin by mouth will resolve issue, but fp should see lumps & get complete history.