thank you for your query,
i have gone through your question, i feel the lump could be because of any cystic lesion causing a lump which is bone hard in consistency(due to expansion of the cortical bone) or it can be a salivary gland tumor
...nothing to worry it looks like benign lesion..you have not mentioned the duration of the lump..
you need to undergo radiographic investigations to rule out any tooth related causes and lesions arising from maxillary sinus
...antibiotics will not reduce the lump..consult a oral maxillofacial physician for opinion and needful..