The HP-100 vanilla protein that you have purchased has not gone off.
The Advanced Sports Nutrition HP-100 is a pharmaceutical protein as compared with other protein supplements on the market which are only food grade. It smells and tastes different than any other protein. It is classified as a growth factor extract, a very potent anabolic protein ultra high in nitrous peptides such as IGF-1 and MGF.
The smell you are experiencing is a combination of a small amount of nitrogen which is being released from the top layer of the protein, because of a slight rise in air temperature, which then mixes with moisture that is in the air forming an ammonia type smell. This is perfectly normal for a pharmaceutical type protein and the protein has not degraded in anyway. When you mix it, it will still taste the same.
It is advisable to keep the HP-100 in a cool place so to limit the smell if you are concerned about it and before you open the tub each time, you can shake it which will also disperse most of the smell.