My horse grew 3 inches taller in almost 4 months, weird?

Did you use a measuring tape or stick? For a real accurate answer I would use a stick opposed to a tape. I always get different answers with my measuring tape. My former TB mare though did grow when she was 4, she want from 16hh to 16.1hh so not a huge difference so yes she grew but not that much. I would double check or have someone help you. Your horse probably grew, just maybe not that much. But if she did, yay for you!!! I love big horses.