Aiyoh, red patches on baby's cheek boleh jadi many things lah, could be eczema, heat rash, or just irritation from drool or rubbing against fabric, especially at 18 weeks when they're starting to explore their hands more. Since it's the second time it's happening, I'd suggest bringing baby to the pediatrician to check it out properly, just to rule out anything that needs treatment, better safe than sorry! A few of my mum friends had similar episodes and their doctor gave them simple advice on what to avoid or how to manage it depending on what it actually was.