If the problem started only after fitting new pads it's likely to be connected. Fitting new pads means pushing the pistons back so if you didn't clean them up first you could have pushed a load of grunge up into one of the bores and a piston is sticking. It's worth taking the pads back out and gently pushing the pistons back out and giving them a good clean and maybe some copper grease. Also by pushing the pistons in to take the new wider pads will raise the level of the brake fluid in the master cylinder so make sure it's not overfilled now. Clutching at straws I know.....
