T5 help!!!! NEED HELP GUYS

I rebuilt my trans (T5) 5 months ago. Its been sitting waiting for the new engine. Today I got it running and the T5 was fine till I got on it. It then wouldn't come out of gear. after I costed down it then shifted fine. It seem everytime I get on it hard it gets stuck in gear. I'm thinking its just stiff from sitting(I HOPE). I just got back form a long drive 30min & about 30 miles and It loosened up a lot and I can now get on it harder than before whitout it binding up. Is this normal with a rebuilt trany? probly not huh !!

thanks for the fast reply....I was full but sitting for 5 months. like I said I went for a drive and now seems better. I drive some more and drain it an put frest fluid in again. Mabey that will help.