Why this price increase?

The other day I went to a pharmacy to buy a 114 ml bottle of Milk of Magnesia. The price printed on the bottle is Tk.13.14, and I had purchased it for that price just a week ago. Now the pharmacy demanded Tk.30.00 for it! However, I went to a couple of pharmacies to double-check and all of them said the pharmaceutical company had increased the price without changing the label. What type of trading is this?