Bubbles forming on skin nearsurgerywound

My father had a calcaneus fracture surgery 4 wks ago.3 wks ago stich removed. Till that once in every 3 days the wound was cleaned using salinewater and spirit and dressed by a medicalprofessional. But after the stitch removal,small bubble like shapes started to form near the wound. slowly it started to spread towards the top side of the ankle.we immediatelyconsulted the treating doctor.they told that there is no infection on the surgerywound and asked to clean the bubbles and wound using saline and keep the wound open.also prescribed antibiotics. It is since 3 days started to take antibiotics. But day by day the bubble like thing spreads and feeling a slight burning sensation. No fever or hot feeling around the surgerywound. also noticed that the bubbles started to spread after the noon.we didn't get to know what it is and why it is occurring .Hence looking for some help from this community to understand what is really going on and is there any thing to worry.no diabetes, takes bp med