Hello, Heather, and thank you for writing. That's a good question, and it has two answers.
First, an FHA mortgage approval is good for 90 days from the time that you receive formal notification of loan approval. However, loans are audited prior to funding, and sourcing your down payment -- making sure the giver of the down payment has the funds in an account, transferred the money to you, and provided a letter stating that the money is a gift -- will be part of that process.
Any material change in your situation -- and the down payment source is a biggie -- means you need to notify your lender and the loan must be re-underwritten. If you get money through a down payment assistance program, for instance, or just ramp up your savings, or crowdfund your bank account, they'll just underwrite on that assumption.
Good luck with your mortgage and thank you for your question.