Can't breathe - chest tightness

Lately I have been feeling very tight in my chest and sometimes feel like I have a hard time breathing. I do worry a lot, and I do try my hardest to control it but I still do. I have 2 little boys , 4 and 19 months, and it seems like since my last child I have had these problems. I have these heart palpitations (sometimes daily and other times maybe 1 a week) and all my blood work has come back normal, I wore a halter monitor and it was normal so I am lost at what is causing me to feel this way. I don't know if it is anxiety, stress, depression or what. I have hypothyroidism and I take 75mcg of Levothyroxine for it so I guess it could be a side effect I do not know. I didn't know if anyone else felt like this and if so what they have done to get better and feel better. I am tired of always feeling this way and I just want to be able to feel normal again. I am 30 years old with 2 beautiful little boys that I would love to enjoy more. I am also afraid to go back to the gym as I had those palpitations while I was there 1 time. I was also told by my doctor that my triglycerides were high last time but my thyroid was also high and that could have caused my triglycerides to be on the higher side. I do not know if all of this has anything to do with each other or not but I wanted everything to be known in case anyone else has experienced this with similar symptoms. I guess that I am looking for answers or ways to relax during the day and just enjoy life instead of worrying all the time. Any ideas/suggestions would be appreciated. Thanks