I totally get what you mean and I find it happens to me sometimes too. It's usually because we generally tend to be different people around different groups (family members, partners, different sets of friends) so when these groups merge it's difficult to know how to act. It's really strange but it's a common phenomenon to feel awkward because people are watching you with your fixed character (as they know it) in mind, so it is more difficult to act any different or act like yourself. This is the beauty of making new friendships and going to new places - you can practically reinvent yourself.
I remember when I was at primary/middle school I was known as the shy one, and that in itself made it very difficult to break out of. However, I ended up finding friends who have never seen me as shy. I'd still feel awkward in front of the original friends though! You need to remember to stay focussed on what YOU think of yourself and your character, not what others do. Be yourself, and be how you want to be. No one can ever snigger or tell you off just or acting unlike how THEY expected.