My GI was a big believer in regular excercise for those with UC and since I was a regular exerciser I was happy for his support. The only time he reined me in was when I had chronic diarrhea. I'm a hiker and with diarrhea one does not want to exercise because it increases motility. So, in your case I would say, no, weight lifting did not cause your symptoms. UC is unpredictable as you will learn and it will drive you near crazy trying to figure it all out.
dx proctitis in 1987
dx UC in 1991, was stable until 1998
1998 started prednisone, asacol, pentasa, nortriptylene, ativan, 6MP, rowasa enemas and suppositories, hydrocortisone enemas, tried the SCD diet, being a vegetarian, omega 3s, flax, pranic healing, yoga, acupuncture, probiotics
2000 lost all my B-12 stores and became anemic
2001 opted for j-pouch surgery- now living life med-free