She was a mother now
By the time Claire went through the stones, Claire had put someone else first for 19 years. The step through the stones was her chance to put herself first. Consistently putting someone else first as a mother changes who you are as a person. She had a daughter in the future who possibly needed her and certainly one she missed with all her heart.
It’s not surprising that when things didn’t seem suitable in the past that she looked for the first chance to escape. Her relationship wasn’t going to be what she may have expected or remembered. If she and Jamie couldn’t be happy, was there any point in staying? There was someone in the future who would be happy to see her.
I can’t even imagine the idea of leaving my daughters behind to go to the past. Granted, they are only 5 and 2, but they’re my girls. If this was possible, I think I’d have a knee-jerk reaction to return to my own time if things didn’t look like they were going to work out at first.
This was a knee-jerk reaction, though. Claire needed time to figure out why she was there and who this new Jamie was. She also needed to figure out who she was after spending 19 years as a mother with a daughter who needed her.