Yes, it is common to pull facial expressions but the wrong ones, or not what others would expect as such. My son looks angry at lot of the time, especially when he has to meet someone or try to converse with them, he looks like he is going to explode. Another aspect that affects this to a great degree is the sheer amount of effort needed to focus on what the person is saying, understand it and try to think of a response. I think the 'angry' face of an ASD person can be one actually of sheer concentration on the current interaction (if that makes sense).
I have witnessed my son 13, helping me at Home Depot for instance, luckily not at a busy time of day... He confirms with me what we are looking for and says he knows which isle it is in, he goes off to look for the product and he does in this visit find what we need, he turns to come back down the store to find me. As I watch him return he looks like he might kill someone if they got in his way, but what is affecting him right at that moment is he is actually happy because he found what we need, but doesn't look it, he is so focused on telling me, he is overloaded by all the bright lights in the store and a fork lift truck has just backed up in another isle an totally startled him! If someone had passed him by and looked at him they might even have thought he was angry at them. He gets to me but doesn't have a happy look on him face, but is relieved to tell me what he found, we quickly make our purchase an leave because by now he is complaining that the 'beeping' from the fork lift truck is really getting on his nerves.
Jamsey, then next time your parents comment on you looking angry, try to think what is going on inside you at that moment. You may even be relatively happy at that point be it is not showing and it may look like anger to them, you may be experiencing some sort of sensory overload at the time too. But sometimes just the sheer concentration needed is enough. When you can get a peaceful moment alone with your parents explain to them what you think is going on and might apply to you, asure them you are NOT always angry but are maybe just so focused and overloaded at times with sensory issues. Give it a try.
My son also had emence problems in school, he look angry most of the time because he didn't understand the other kids and the noise level drove him crazy. Things are better now for us because I have realized what is setting him off or triggering these looks. Try to give your parents some insight into YOU as they don't realize what is going on. 