are you doing the proper warming up cycle of stuff and then repeating it afterwards to warm down?

sometimes the aches can be due to the build up of lactic acid in your system that you haven't flushed through, the same thing that causes the burn

but i honestly think it's probably just a case of starting to heavy bceause youmight be embaresed that you can only lift a certain amount of weights

don't worry, i can bench about 25 kgs and i've been doing it for ages