Q: little dog getting aggressive?

well little lily is a shih-poo and she is just over six months old.........shes been fine up until 2 weeks ago when out of nowhere she snarled and snapped a me very aggressively for no reason at all, it has happened more and more since. I have treated her with nothing but love an respect since we nursed her back to health from kennel cough 5 and a half mnths ago....we realise she may be coming into heat and have the op booked 2 weeks from now...also we recognise she may be protecting mummy( my partner) would just like some pointers please people???? daddy( jez) who loves lily very much,