Hi there, yes I have had this problem. It's usually a fungal infection (yeast, thrush) - the bugs are on our skin all the time anyway, but if our body gets out of balance the number of bugs can grow and we get an infection. Thrush is common in pregnancy even without diabetes. Any kind of diabetes can make it more likely.
The cream is often the best treatment and it can work fairly quickly but if you still have pain or discomfort please go back to the doctor to see what else can be done.
If it's not a fungal infection it could be something else, which often is easily treated too. I'm not talking about STIs but irritation or infections of other types. I've had those irritations too. Any problems "down there" can be very painful and stressful.
Congrats on your pregnancy, by the way!