I installed a Crosswater concealed shower valve and connected in to my Crosswater ceiling shower head with 15mm coper pipe. The pipe runs vertically up the bathroom wall then turns horizontally above the ceiling in the loft and down again to connect to the shower head. About 3-5 minutes after having a shower, water comes out of the shower head in a drizzle for about 20-30 seconds and it then stops. It happened only once, every time after i had a shower. Initially, I thought that the water coming out collected in the shower head but then it happened once when when the shower head wasn't connected. Any ideas what's causing this anomaly?!