Over emotional?

Today is one of those days that for no reason at all i am super sad and Every thing is just kind of gloomy.... I'm 28weeks, nothing made me upset i actually had a pretty good day sun was shining my significant other was in a good mood, i had a Canadian friend visit that i don't see often and i should be completely grateful for all the blessings i have in my life... Roof over my head, hard working man (i don't have to work).... sitting here on the cell phone he pays for i feel like i should be more grateful but I'm just sad.... Please shed some light other moms and tell me I'm not just crazy... I know I'm pregnant and that comes with more emotion i just don't feel like it's an excuse.... Or is it?