Some chins just don't like hay. I would keep offering it; she might change her mind. I have had some who did.
The only way that you can be sure about the pregnancy is to wait. count 111 days for the day that she was last with a male (if you don't know, count from the day you got her). If she does not have kits in that time, she was not pregnant. I would not take her to a vet to find out. It would be too stressful for her, especially if she is pregnant.
There are some classic signs you can look for; large, hard belly, weight gain, elongated nipples and hinging out at the bottom of the cage. You might also feel or see the kits moving. Most of these signs won't be evident until the last month of the pregnancy, if at all. I had two girls fake me out recently. They both gained weight as if they were pregnant for about a month to a month and a half, then leveled off. Neither one was pregnant. I have also had 2 girls who showed no signs at all and were pregnant. The presence of kits proved it.