Hi, I think they can come into season earlier especially if kept indoors and it's warm. The time for females to come in to season is a lot earlier and over a longer time than most books say, i think this may have been based on just keeping chipmunks outside.
A female chipmunk can often be unfriendly to other chipmunks she is normally friendly with, especially males. She may want to eat a lot of food she doesn't normally eat. I had one which would eat worms when she was pregnant. You might also notice a pregnant female will start using mineral blocks each day. They tend to remain just as active, climbing and jumping nearly to the day they give birth. You could also notice increased nest building as well.
Very often you will notice an increase in weight, younger chipmunks seem to have larger litters, and larger litters in the spring rather than the autumn. The babies are carried inside just under the two hind legs. Towards the end of the pregnancy you may also notice a row of teats underneath her, there is a photo of this on this page, near the middle:
http://www.super-chipmunks.co.uk/breeding.phpHope anyone who has breed chipmunks might also come in with anything strange their females did during pregnancy.