Why is the internet in my room so bad?

For some odd reason, the second I walk into my room, the internet becomes very slow almost to the point where it is unusable. Literally when I walk inside the door, it sucks, but when I walk out, it's fine again. It doesn't make any sense. I got an airport express and I use it as a range extender for our airport extreme, but it doesn't help much. Does anyone know why this happens? My dad told me that it could be because I have so many different things (wireless printers, speakers, my phone, ipad, computer, ect) in my room that it interferes with the signal but I don't think that is the reason why.