I believe the problem you are having with phlegm and the lump above your tooth are not related. The fluid filled bump is most likely do from a tooth infection
. Usually a chronic tooth infection will begin to drain in the gum tissue above (or below on the bottom teeth). If this is the case the treatment would be either a root canal
if the tooth is still restorable or extracting the tooth. The tooth infection should be addressed because it can lead to a larger infection.