Hi, hope someone can help. About 2 years ago I plastered the kitchen. I have recently noticed some wall tiles cracking so I removed them to find the bonding coat bulging. I have never seen anything like it before. It was perfect when applied and then skimmed. It’s has three bulges/ lumps about 4 inches in diameter and sticking out about half an inch, and hollow behind. Has anyone seen this before and what could have caused it?