My fiance currently has Medicaid. Will she lose it when we get married?

This is typically as State issue. An elder law attorney may provide advice that will permit your fiance to continue to receive benefits despite becoming recently married. In particular, there is the issue of countable resources, if you open a joint account this could put him over the limit. Also, you need to be careful about assets your fiance might inherit if you were to predecease her.
Typically, when a married couple has one spouse apply for benefits, after the Community Spouse Resource Allowance has occurred, the Community (healthy) spouse's assets can grow without affecting the others eligibility.
Also, in most states, the community spouse's income cannot disqualify the eligibility of the other.
Find an elder law attorney in your state. It sounds like you need advice and perhaps some planning to avoid problems.

Seriously consider breaking or delaying the engagement until you can find an attorney who is familiar with the Medicaid pitfalls. All income/resources as well insurances get counted for applicants and spouses. Spousal impoverishment rules may apply if married.
If single, she has one set of assets/resources in the equation. If married it will be determined after adding and later dividing individual income and resources for each of you.
From what I've learned so far, when we are young it is usually advantageous to marry and combine resources to get ahead in this world. Middle age, maybe yes/no. Older age - extremely questionable. So it's likely best to carefully check this out before you marry.

Check with Medicaid. There is a spousal empoverishment exemption which may apply. Is your fiancé elderly? An elder law attorney that specializes in Medicaid planning would be able to answer your question.

Very possibly, if your combined income is over the limit.