The Alpha dog/dominance theory is very outdated and when you think of it, doesn't make much sense - why would your dog suddenly think she's a human?
It could be that growing up, she's grown tired of playing with other dogs, or with some specific dogs' type of play, but that's no reason why she'd get upset at the mere sight of another dog. A couple of other options you could look into:
- she hasn't had much contact with other dogs while you were away (without regular socialisation, dogs tend to turn into hermits),
- she's had one or several bad experiences that haven't been counter-balanced by good ones. If she got into a fight, had another dog snap at her and scare her… It's easy to miss out on those moments at the dog park, even if you're supervising your dog.
In either case, I'd definitely try and reintroduce her progressively to nice dogs that you know and trust and with whom she could rebuild her confidence, encouraging her with praise and even treats when she remains calm. If she gets nervous, take a step back - don't force her to leave her comfort zone - and reward her for any moment she looks in your direction, even briefly.
If needed, ask a professional for some help. I hope she gets back to her old social butterfly self in no time!