Assuming nothing happened that is obvious, you were probably just interested as long as you were trying to get her. Now that you have her, there is nothing to excite you about her. Fairly shallow on your part, but most guys are the same way.
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Some of the time it is due to ED, might want to check with your doctor about the problem if you are having it all the time.
Also Foreplay could help achieve erection
Its normal because you have just met her and probably don't have a sexual feeling towards her yet.