What causes frequent & easy bruising?

Hi. I realise recently I am bruising very, very easily. Two months ago I went on an Outward Bound trip and both my legs were nearly completely bruised. My friends asked if I had any blood related diseases when they saw it. Today I notived that my wrist bruised after I use it to carry a shopping bag which wasn't very heavy. This spurred my curiousity if I should worry?