Similar Situation
Well my girlfriend always seems to be the same, and it seems to be more that she is uptight.
Two things i found really helped.
1. Just relaxing, and giving her back, head, feet, leg or bum massages and just taking time to relax her, and also perhaps making it fun by perhaps mucking around with body chocolate perhaps.
2. The second thing we tried that really seemed to work was have a shower, spa or bath.
The main thing is to make it so that no matter what she doesnt feel self conscious about it, and that you reassure her that its okay.
Let me know how you go.