tacoma mt 1st gear issue

Ive had my tacoma for around 4 years bought used, I replaced the tranny at around 190K miles and now sometimes it comes out of first gear. Not like the old one that I replaced, its more like it slams out of first, and only sometimes after a dead stop, I really dont know what it is, hoping to hell its not the transmission, since it feels like something hits behind the tranny.