Hi Tricia...good work so far! Well done! My first thought is that you may have improved to the point were you are running quicker than you did to start with. It might just be that the speeding up is a little too quick so isn't sustainable for the whole run, so slow down a bit, relax and breathe steady I would imagine that will see you through. It might also just be a sign that you need a little break. Try a swim or cycle instead. Also when varying your runs try some hill sessions which will build strength and stamina, and some fartlek sessions where you run at a steady pace, then speed up, then slow down this helps develop different capabilities for your body. Overall just enjoy it ! 

1. Just a thought but do you think you could be suffering with the high pollen count. I have never suffered before but this year found myself struggling to breathe on my hard runs and races. My training one day would be fine and then two days later I was struggling to complete them. I started to take one Pireton tablet each day and found my training improved. And last Monday I forgot to take one and found myself at the back of the group sweating and finding it a big struggle to keep up, but no other symptoms you could pinpoint it to. It’s like running at high altitude you know something is wrong, but if you didn’t know the air was thinner you would think it was down to training or illness. Roll on the winter LOL.