apple watch series 4 charged overnight but showing red lightning bolt

Recently purchased a series 4 LTE Apple Watch which has been working perfectly - last night I put it on charge as it had died and expected it to be charged come this morning, woken up and it’s just showing a red lightning bolt within a white ring? It’d been charging for roughly 5 hours when I first woke up and noticed the issue and a further 4 hours whilst I left it to charge thinking I’d not put it on the charger correctly - help!