She seems uptight post-intimacy.
Unless you’re both in rush to get somewhere and just settled for quickie, most people need a recovery window post-intimacy. If she really hit her peak, her muscles will relax just like yours do. If she’s flopped out in the bed with a smile on her face, she likely did not fake it. If she’s uptight and stiff, she may have faked it. The more you pay attention to her body, the easier it becomes to read her over time.