When you are making good progress with your tremolo exercises, there is another one that you can try if you are feeling adventurous. As well as practicing the normal way, you can also try reversing the finger direction (i.e. fingers plucking away from you instead of towards the guitar or soundhold.) My understanding of what this does is it exercises the opposite micro-muscles in the fingers and therefore further develops your finger strength. To begin with the reverse action will feel very awkward and you might feel totally frustrated and give up. But if you persevere until you can do a reasonable job of plucking 'in reverse' (note that it doesn't have to be fast but it must be played evenly) when you return to the normal direction you should notice a difference. The normal tremolo should feel easier.
As with all technical exercises, if you feel any pain at all please, please stop practicing immediately. Put your guitar down, go and do something else. Give your hands (and brain!) a good long rest before starting again.