Why my girlfriend seems less excited to see me after months?

We're neighbours and 2 years relationship. She studies in another state so we only see once in 2-3 months. Her family doesn't know about our relationship. So I cannot talk to her openly when her family is with Her.

Today morning i went to her house and saw her for 5 minutes after 2 months. Yes, She was excited. Then later we met them at a cafe for 45-60 minutes and she only looked at me once or twice. That too without an interest. I asked about it now and she said it was post period mood swing. I've never seen such post period thing on her.

What do you think?

Why my girlfriend seems less excited to see me after months?
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