I am a professional gardener and it looks like from the photo your begonia is suffering what we call elongation due to lack of light. It causes them to grow all lanky. I suggest you eithe remove the shade cloth or put the plant in a position where it receives morning sun and afternoon shade. Mine is in a pot and it gets the morning sun and afternoon shade. We get really hot summer days here in Melbourne like over 40C (104F) and then I bring it inside and we all hibinate together.
The shade cloth is blogging out too much light. A prune back would also help as BewilderedGreenyO.o suggested.
Cheers Keen Gardener
Interested in growing vegetables, annuals, herbaceous perennials and succulents. Love historic gardens and am involved with the Friends of La Trobe's Cottage in Melbourne in the garden group.