Phlegm in throat is starting to ruin my life.?

For the past couple months I've been noticing a mucous like build up in my throat. I always feel like there is something in my throat and am constantly clearing it. It really gets severe after I eat. About 5 min. after I eat a meal I get a lot of phlegm built up and I have to cough and gag for about 20 min. untill I get it out. It subdues back into just a feeling of something in my throat after that. I've always had really bad allergies so Im sure it's sinus or allergy related. I went to the doctor and got a steroid shot for it and he prescribed Flonase to me. I really need more help though, I want to be normal again and be able to workout and stay up on my fitness like I use too but with this phlegm it makes it tough. I cant physical activites without getting out of breath easily and coughing and hacking. I have trouble even playing pick up games of basketball and stuff like that. This phelgm is really starting to take a toll on me. It doesn't help I have anxiety too. Anyone else experience this sorta thing or can help? Thanks. p.s. Im not a smoker and am fairly healthy.