Not sure if my tarantula is in pre molt or not(G. Rosea)?

I just got my first tarantula at Petco, what I noticed was that there was a web spun on the ground but the tarantula was in her hide. I thought she must've already molted. However I brought her home and she is really sluggish, she just stays in a corner. When I went to bed s/he was in a corner, I wake up and she's still in that same corner. I looked at her underbelly and it is pretty dark, I'm not quite sure if I should feed her or not. S/he's about 3-4 inches. Thanks