My cats purr sounds nasally, like she has a runny nose?

My cat Bella is a grey and white domestic short hair. I put her up on my bed, and she started to nest (you know, where they look like they're padding on the spot when they get comfortable), she started to purr unusually loud, so I had a listen, and it sounded a bit odd, like she had a runny nose or some phlegm in her throat, because she swallowed a couple of times. She's only 3 years old, so she is not old. She likes to go across the road into the neighbours garden and disappear under big leafy bushes. Does she have an allergy or something? It is winter time, and it is very cold outside and Bella does not often come inside at night, could it be the cold? Vets, people with cats, anyone who has experience, please answer? :3