Late April is too soon to go outside, mother natures cycle of light change for Cannabis is May 21st through August 21st, anything growing outside before or after has a high probability of going into the flowering mode. If you're plants are only 20" tall and have been planted since late April that seems awfully small to me and my guess is they went into budding shortly after you planted them which is why they are so small. It takes a LONG time for a plant to pull itself out of flowering and back to vegging before you notice any significant growth. IF you had a lot of overcast or rainy weather that could cause them to trigger as well.