The very fast development of quantum computing is a serious threat to traditional cryptographic solutions that are used in wireless sensor networks (WSNs), especially those that have severe energy and computing requirements. The paper presents a lightweight post-quantum cryptographic (PQC) system that is specific to the resource-constrained WSN. The aim is to provide quantum-resistant security with a minimum energy requirement, latency, and memory overhead. The solution proposed combines lattice-based primitives, optimized key generating schemes and compact encryption schemes, with mathematical modeling to validate their performance. Experimental assessment shows that the framework has a security against quantum attacks, consumes less energy by 28.4% and latency is less by 31.2% than traditional PQC instantiations. The framework also ensures scalability with heterogeneous sensor nodes.