Started antibiotics for sinus infection 3 days ago, now i have a bruise on my stomach that is dark purple and spotted with red. should i be concerned?

What antibiotic?: What antibiotic were you placed on? Did you accidentally bang your stomach into a sharp object?

May be: Many medications can cause panniculitis or polyarteritis nodosa, especially in at risk individuals with concomitant hepatitis b or c infection or HIV infection. The condition is falls under spectrum of vasculitides, which can be precipitated by immune complex mediated process. Any abnormal bruising, bleeding tendency, purpura, skin rash, or skin ulceration needs further evaluation.