Breathing help

Alright so I'm having a problem with my breath and singing. I'm not sure why by but after singing for awhile my nose starts to feel tight like its closing in the middle. I keep on singing then but then feel like there is mucus and saliva or something running down the back of my throat. Then when I go to take a breath I get all messed up and cough or can't take one or its very small and I sound like I'm singing with mucus in my throat or almost like I'm holding my nose. At first I thought it was a cold but it has lasted for a year now and I can't figure out if I developed allergies or something else. Can anyone help on this matter to stop the stuffed nose sound I'm making? Thanks