Yep, I'm a Type 1 and go to the gym. When you exercise your muscle cells increase the number of insulin receptors at the cells surface. This means you need less insulin to carry the glucose into your cells. Therefore, if you don't decrease your dose of insulin, you will certainly get hypos.
Consider buying the book Think Like a Pancreas, it has got tables in there that let you calculate the change in insulin dose according to duration and intensity of exercise.
I adjust my Lantus and my Novorapid, but I'd start off just adjusting your Novorapid first. Stick to one variable at a time. If I eat after exercise, I decrease my Novorapid by half.