Why does my cat hiss now?

My female cat has suddenly picked up the habit of hissing at anyone who comes over to our house. When she was a kitten, she was social and used to crawl on anyone's lap and snuggle with them, purr, lick them, etc. She was the most affectionate cat in the world. Now, for last 4-5 months or so, at almost 2 years old, she hisses at anyone who tries to pet her and then runs away. They won't even walk up to her and try to pet her in a threatening way - They'll just be sitting on the couch and she'll walk by them and they'll just pet her as she's walking by and then she'll react. She seems to get more agitated when people sleep over as well - At this point, I'm assuming it's a territorial thing and she's just not happy with other people on "her property". She just never used to be like this though. It's very strange and it seems like she just picked up this behavior out of nowhere. Now she's even started hissing occasionally and me and my boyfriend when we pick her up and she doesn't want to be held. We don't think she's in pain or sick or anything because she does still let me and my boyfriend pick her up and she'll often come over and lay with us. Could there be any other reason for this behavior besides her being territorial and just easily agitated? What can we do to stop this behavior before it gets worse? I just want her to go back to her sweet, affectionate self. Thanks!