No, just because you got your period doesn't mean you lost weight.
The problem with PCOS is that it makes our cycles wonky and something happened in your body that it said, "Ok, go release the cycle!" but it could be hormonal in nature, not weight loss. It could be anything, in other words.
If you haven't seen a doctor recently, you should -- not because you got your period, but because if you have PCOS and you're not treating it, you may be losing the windows of opportunity to get better and healthier.