just on a guess your paint was too thick... it dried before it could level out.. try dipping your brush in thinner next time, wipe the excess off then put the paint on it... or if you ALWAYS brush it on, thin the bottle of paint down a bit..... it might take several coats to cover it that way but there will be less chance of it leaving brush marks..
But like MM said, if you could give us some details like what brand of paint, and better yet, pics of the inside it would help us narrow it down even more!