Initially I thought yes, you would be a citizen by birth, but after looking at the details closer, I don't think you are able to claim the citizenship unless you have contact with your grandfather. The parent has to have lived in the USA for a certain period of time as described below. So unless your mom lived in the US for at least 5 years, including a few years as a teenager, she is not able to pass on her citizenship to you. The USCIS page also talks about grandparent physical presence being sufficient. So if you can prove your grandfather's residence and citizenship in the US, and his connection to your mother and her citizenship, you may be able to claim the US citizenship.
The law in effect at the time of birth determines whether someone born outside the United States to a U.S. citizen parent (or parents) is a U.S. citizen at birth. In general, these laws require that at least one parent was a U.S. citizen, and the U.S. citizen parent had lived in the United States for a period of time.
A child’s U.S. citizen parent must meet the following physical presence requirements:
The parent has been physically present in the United States or its outlying possessions for at least 5 years; and
The parent met such physical presence for at least 2 years after he or she reached 14 years of age.
https://www.uscis.gov/citizenship/learn-about-citizenship/i-am-the-child-of-a-us-citizen
https://www.uscis.gov/policy-manual/volume-12-part-h-chapter-5#:~:text=A child's U.S. citizen parent must meet the following physical,reached 14 years of age.