Watermelons fruit drop

Hi all. I have the watermelon 'Little Darling' in my greenhouse. The plant is happy and I and the bees successfully set three fruit. The fruit started to swell to the size of a large olive then shrivelled. I have a solar watering system and feed weekly with tomato fert so I'm not sure what's happened here. It did get very hot on Friday though I left the door open, as well as for the bees and hoverflies to access the flowers. Any ideas?

Thank you! I had a look through the posts and suspect the high daytime heat that the closed greenhouse was getting put too much stress on the plant. It's in a huge pot, on an automatic watering system with weekly tom feed and pollinated, so by process of elimination the heat is the problem. Next year I try a different placement.