No, it wasn't by chance; if you had bought credits before today (and possibly before a certain time today), you would have gotten X credits when you purchased them and another X credits sometime today.
This would be buying credit packs though, not subscriptions.
will the credits we got from subscription be reset too? I suppose it doesn't make much difference to me (except for the "extra" month I'll get) since its been almost a month since I bought my subscription . Still, I'm curious to know what date my formal subscription will begin on.
If they've doubled the credits from credit packages already, I would assume that they've also officially started the clock on subscriptions. I'd recommend going into your Account page and checking there to see if the date has changed from when you last saw it.