I think the OP has been given a bit of a rough ride here.
Yes, the OP is a tad possessive of her son, but faced with that kind of intimidation from a 'been there, done that, I know it all and what's best' MIL I'd be possessive too, in fact I was with my DS. I know what it's like to be undermined by your in-laws whilst juggling them providing childcare, it's awful.
OP's MIL calling her breasts 'pillows' in front of other people is weird and, to me, a show of possessiveness in itself.
Feeling you need to get a backbone, and quick. This will only get worse as time goes on. You are your DS's mother, and you're a tad possessive but, I think, with good reason. If your DS is allergic to something in your MIL's house then you need to make alternative arrangements for him to be looked after in a different environment.
Family providing unpaid childcare is always tricky, as the unpaid element means there is a sense of entitlement to comment on aspects of how good a job you're doing and areas you're lacking in. Be firm but fair, and backing of your partner is key.