Hensinthehedgerow, I am a second-year student midwife, studying in New Zealand, and first of all I would like to apologise in the name of every midwife and assure you that your feelings are absolutely valid, and that what happened to you was absolutely wrong. The midwife acted in an unprofessional way and needs to be called on it. She will benefit from going through an informed consent-informed choice course.
Second, I will try to give you some information so you know what should have happened to you. If you gave birth to your baby (congratulations, by the way :) in the water without pain relief, the first thing I must say if a big WELL DONE to you, what a fantastic way to welcome your baby to this world! After birth, especially when the midwife has not had a clear vision of your perineum, it is necessary to perform a perineal examination of the internal vaginal wall, labia, perineal surface and anus to check for possible damage, in the shape of grazes and/or tear, and if there is any bleeding, to ascertain the origin of this bleeding. The midwife should have explained this to you, and also told you that, after birth, that area is particularly sensitive, and often this examination is painful. After the explanation, she should have asked whether you understood, whether you had any questions (and answer them appropriately), and asked if you consented to the procedure. Only if you say "yes" then she should offer you the use of gas&air (Entonox) if you wanted, and ask you whether you were happy with your partner in the room or you'd prefer to be alone. Then she should prepare a sterile pair of gloves and some gauze and do the procedure, reassuring you that she will immediately stop if you ask her to. After the procedure is finished, she should have explained to you what she found, and how she is going to follow up.
During our studies, informed consent and choice are literally drilled into us, and yes, performing a procedure without consent, even if it's just taking a blood pressure, IS assault. Being in a hurry of understaffed is NO excuse.
I do encourage you to complain, and I personally recommend you to ask for a meeting with the midwife so amends can be made, and so you can express to her exactly how her actions made you feel.
I hope you recover from this as soon as possible and can move on and enjoy your baby :)