Motherly Behavior?

My cat gave birth about a month and and she has been fine until now. For the past couple days she has been acting differently. Her baby is kept in the basement in the back corner, but now when we go downstairs she sits near the stairs and hisses at us when we go down.. But right after she hisses she meows like she always does.

Also, she runs up the stairs now and runs past us on the main floor like we are scaring her, but we are not doing anything differently ..She does it like something is scaring her.
Earlier today she came up the stairs and from where the door is you can see straight into the kitchen, so she walked towards the kitchen and hissed at my grandma, and my grandma wasnt even aying attention to her she was by the stove cooking something.

She has been acting very on edge lately, does aanyone know what it might be from?

As always, if there's a change in behavior it's a good idea to call the vet.
I've never had a mother cat change behavior like that - either they start out grumpy and protective and stay that way, or they just don't care. BUT, I've been lucky enough to only have been through that ordeal about 9 times.

You may find a little more look searching through the pregnant and kitten section.