I've been feeling sad and idk why?

Lately I've had the most severe mood swings. I'll feel so very happy and overjoyed with life and loving life loving everything (this is my usual self) but the last few months or so I've been so so so sad and I'm not sure why. I kept blaming it on my period but it's been much longer than that. There's some things I've been upset with lately but I mean I'll be fine and just overthink one small thing and I'll get so frickin depressed. I'll cry so hard and not have a valid reason behind it. I'll just genuinly look depressed and everyone knows me as bubbly and energetic so everyone's been noticing too. But then sometimes I'll get like that again but then get beyond sad later that day or maybe the next day and I don't really know why. I've had things on my mind lately but not enough to be THIS sad about and for this long. I feel incredibly alone and unloved and I know I'm not but I just feel that way. I've also had this thing where my heart rate will speed super quickly and then I can't stop moving I'm just so jittery and tense and I can't control my breathing I hyperventalate and I can't speak it's when I get so overwhelmed and so beyond sad that my chest actually hurts a pain. Anyways I don't really have a direct cause on why I've been so sad like what's up why does my mood keep doing this