It is not uncommon for a 2 and a half month old baby to experience constipation, especially when changing formula. It is important to check for any alarming signs such as fever, blood in the stool, increased discomfort, excessive crying, or reduced intake of milk. If any of these signs are present, it is recommended to seek medical advice. Otherwise, you can try increasing hydration by giving small amounts of water throughout the day or gently stimulating the anus with a finger or a lubricated object. If the constipation persists, a glycerin suppository or a stool softener may be helpful. Be sure to consult with a pharmacist for proper guidance and dosage instructions.