It's just the hormonal change, it happens to me.
This has happened to me many times throughout my pregnancy. Its to the point my husband and I now have lube for just in case.... I have no idea whats up with it but im just glad im not the only one... its a bit embarassing when it happens T_T maybe hormonal change?
I wish i knew the answer to help im sure its normal for your body to go through many changes like this