Chemo is systemic, and can cause peripheral neuropathy (though this is generally cisplatin - not Erbitux which is not really considered to be a chemo though it is definitely systemic) so I can see how it would effect you and cause drop foot (radiation mat be the culprit too - depends on where that beam is aimed), this early on I would get to physio as soon as possible to try and prevent it from becoming a long term issue. If it has been caused by radiation (much like l'hermittes sign - it may only be temporary and will subside as your heal, but I would still push for physiotherapy.