Money gift from a non-US person

Your mother is not a US citizen, nor is she living here. She would not be subject to a gift tax from the IRS. She needs to check with the Bulgaria tax authority. You do not even need to report a sub $100K gift from abroad. See

Since your mother is not a US citizen, gift tax would only apply if the money she sends you is located in the US at the time she makes the gift. In other words if she has a bank account in the US and makes the gift from those funds, the gift tax will apply. Otherwise it won't.

Joe is correct that you don't normally need to report gifts from abroad of up to $100,000 (that's the total gifts over the course of a year). However if the gift came from the account of a corporation or partnership, it has to be reported (IRS Form 3520) if it's more than about $16,000. This is just a report - no tax will be due.

Gifts are almost never subject to income tax. So that should not be an issue.