Sudden, overwhelming feelings?

Evening all. I'm an anxiety-girl normally, but do occasionally dabble in bouts of depression which I can normally manage to plough through!
I just wonder if anyone can relate to my latest situation. I'm a happy lady - love my job, boyf, friends etc. I'm a positive thinker and I accept that I'm not mentally 'great' and try to plod on anyway.
Lately though, it's like I'm plodding on doing a nice thing, like having a meal with friends or snuggling on the sofa, and I get a huge, overbearing feeling of HORRIBLE. That's the only way to describe it, like "everything is pointless, you're not ok, just get in bed and put the cover over your face and stop pretending to be fine" and I can't ignore it. My lovely day is then ruined because I can't stop thinking I'm not ok, everything is awful, I can't deal with life. Then I'll wake up in the morning - it's a new day, life's good, I'm pottering about and BAM it comes again. Is this normal??

Are you under more stress than usual? Sleeping and eating properly etc? I think it's normal to some degree to have these feelings without being "depressed". I don't consider myself depressed at this moment in time, but if I'm under more stress I can have hopeless feelings.