What Causes Scanty Menstrual Bleeding?

My period was about 3 days late. I had my period last week, it was spotting colour (brownish discharge) for like 3 days then I had one day where I had little blood but for the whole 4 days in total it wasn't heavy like a normal period is..I was in discomfort and feeling sick through out the day and started to bloat but anyways I finished 4 days ago and started the pill again. I have never had a period like that before. Now I'm still in discomfort and left feeling sick through out the day with slight tender nipples and little bloated still.. But I was on antibiotics back at the begging off January for chest infection before I started my pill again..