Is this normal?

Yes this is completely normal and generally begins with the lower and older leaves. What actually happens, is, that cannabis being the super plant that it is, will draw upon the growth elements, such as sugars, amino acids, carbs etc that are in the older leaves for re-use during the flowering cycle. It is far more efficient for the plant to re-purpose/recycle the growth elements from the older leaves rather than making them from new and growing flowers at the same time. This way there is more energy for flower growth without some of the energy having to be used for making new carbs, amino acids, sugars etc. This is one fact that the defoliation advocates fail to understand, that by removing healthy green leaves, not only does it reduce the plants ability to produce energy, it also robs the plant of this wonderful resource of ready made growth elements she needs to grow her biggest and best flowers. Hope this helps,...... Organoman.