The advancement of healthcare technology, along with the integration of the Internet of Medical Things (IoMT), has improved the efficacy of patient treatment. However, IoMT devices continue to encounter significant security challenges, such as data tampering and unauthorized modifications during data sharing, which threaten the privacy and integrity of medical records. Although advancements in blockchain and cryptography technologies, current methodologies inadequately address these difficulties. To address these security gaps, this paper introduces a novel security framework that leverages lightweight cryptography and blockchain technology for safeguarding Electronic Health Records (EHRs). The proposed approach employs an Improved Merkel Tree (IMM) hashing technique for data integrity, an Amended Elliptic Scheme (AES) for secure key generation, and Lattice Homomorphic Re-encryption (LHoRe) for data encryption. Key optimization is performed using the Opposition-Based Coati Optimization technique, while encrypted data is stored in the Inter Planetary File System (IPFS). Performance evaluation shows that the proposed framework achieves a high average trust value of 98.75% and reduces data retrieval latency to 2.24%, offering a more secure and efficient solution for IoMT data sharing compared to existing methods.