Think I missed One - Obstruction at 1B

[fastpitch]

Can you call obstruction at 1B if there is a fielder behind the safety base that causes the runner to slow down?

Tonight in a 12B ASA Tournament had grounder to 1B, 2B covers 1B but 1B decides to take it. 2B moves over behind the safety base while 1B tags the white base on a close play before the B/R touches and I call her out. The B/R slows down due to 2B being behind the safety base, maybe 2 feet. She would have run over the 2B had she continued through the base. I think I should have signaled obstruction and awarded her 1B. I haven't reread the Obstruction rule tonight but would appreciate comments.