In this paper, we present a single-trace attack on a BIKE Cortex-M4 implementation proposed by Chen et al. at CHES 2021. BIKE is a key-encapsulation mechanism, candidate to the NIST post-quantum cryptography standardisation process. We attack by exploiting the rotation function that circularly shifts an array depending on the private key. Chen et al. implemented two versions of this function, one in C and one in assembly. Our attack uses subtraces clustering combined with a combinatorial attack to recover the full private key. We obtained a high clustering accuracy in our experiments, and we provide ways to deal with the errors. We are able to recover all the private keys for the C implementation, and while the assembly version is harder to attack using our technique, we still manage to reduce BIKE Level-1 security from 128 to 65 bits for a significant proportion of the private keys.