Feeling misrable

Do you ever get an overwhelming feeling of depresion just wash over you, for no particular reason but it suddenly just hits you. Thats how i feel at the moment and i dont really know why, i am so anxious i actually feel sick but also so very low i want to cry.
Its just so odd as things in my life have been going well for me, i am under a lot of pressure but thought i was coping ok. I feel like i have no friends right now i feel rather lonley i think maybe im really selfish and i am expecting to much from people around me.
I do suffer from panic attacks but have learnt to control them but just lately i have been having one most nights although i can control them it makes me wonder why they have come back when it has been several months since i last suffered any.
Thinking about it, it feels kind of like when you have been hiding behind a smile making out that everything is ok when infact its shit and then the moment comes when you cant hide it anymore well thats how i feel but i dont understand why.
This doesnt make much sense i know sorry.