Hi Radica,
Astragalus can make a subclinical inflammatory response to a latent pathogen become clinical. It ramps up antigen presentation and the subsequent immune reaction, making it become evident.
In Chinese medical parlance, this is termed 'trapping the thief inside the house', as Astragalus is effectively considered an astringent, by reinforcing Wei Qi. Essentially, Astragalus 'bricks in the windows', preventing the pathogen from being expelled.
To prevent against this, Astragalus is seldom used in Chinese medicine alone, but often with Wind dispersing herbs. An example is Yu Ping Feng San (Jade Wind Screen Combination), to which you could further add Bai Zhi (Angelica dahurica) to direct those expulsion efforts to the nose in particular. This potential aggravating effect of Qi tonics is also the reason for the Chinese medical dictum 'Never tonify an Excess'. So we remove the Excess (Wind pathogen) first, before jumping in there with the (Wei Qi) tonic.
One other idea is to look at this as a Blood deficiency and Blood stasis condition. Blood deficiency is aggravated through summer, and can lead to viscous secretions while aggravating Blood stasis (i.e. chronic inflammatory) conditions. So if Yu Ping Feng San doesn't work, you could give Xue Fu Zhu Yu Tang a look. It has an affinity for the upper burner, which is definitely the location of the problem.
Hope that helps,
Steve