Why is my baby so clingy all of the sudden?

My son is 8 months old. As of last week he was happy to sit and play for a period of time. It was always long enough for me to do things like vaccuum, cook dinner, email, etc. This week he absolutely refuses to sit up...he stiffens his legs. He also refuses to not be held. No matter what I do-put him in his bouncer, on his playard, in his pin, he starts crying and doesn't stop until I pick him up. He is completely fine then. I have tried not to "spoil" him...and as of last week he wasn't. I have tried spending lots of time playing with him this week and whatnot but he hasn't let up. He screamed at me the entire time I made dinner last night, I need help!!!!