Hello, and welcome to the forum
Firstly, with bitches, it's called a season, and can be referred to as being 'in heat' or if you're from Yorkshire, 'breaking down'. To be honest, there's no rule about when a bitch has their first season, and it varies tremendously, so until she actually starts to bleed you may not know. Signs can include changes in personality, swelling to the vulva, and an increase in leaving scent marks when walking. That's just three things I notice with my girls, but there are any number of changes when a bitch is coming into, and comes into season, and all are different.
If you want her to stay on your bed, buy a robust blanket of some sort, a lot of bitches are particularly clean in season, and you don't notice much blood, but there are some who do loose large amounts of blood unfortunately. Let's hope your girl isn't one!
I would also keep her in your sight at all times, she could get caught out, I don't walk mine at all when they're in season, as I don't think it's fair on other dog owners, but then I have the luxury of living in the middle of nowhere with a big field next to me I can exercise them on as well.