Toddler hitting out!!! Advice needed!!!

[Vans84]

Hi there, I have a 21 month old daughter who won't stop hitting other children. There is no reason for it, every play group we go to, a child will walk past her and get hit. I have tried removing her from the situation and explaining its not nice to hit. When at home she has hit myself and my husband. I've tried the naughty step but she will always find something to do. Over the last week I have put her in her cot in her bedroom for up to 10 minutes and I know it's early days but still has had no effect. I don't know if that's the right thing as she is really good when going down for a nap or at nighttime so I don't want her to think her cot is a punishment area. I find it extremely embarrassing when out in public. I have recently had a second child so that may have triggered the behaviour but it has been going on for 6 months!! I know children go through these phases but it's got to the point now I'm finding it upsetting. Any advice would be gratefully received. Thank you