At least since I began working in paediatrics around 2015, vaginal examination (any kind of internal examination) is not conducted in prepubescent children except in exceptional circumstances and usually under GA. It's recognised that VE may be traumatising for children and often adds very little to the clinical picture.
However it is sometimes necessary to examine vulva (always with a chaperone, usually just looking not touching). Some relatively common reasons I can think of to look (and I mean literally look) would be trauma (think falling from a bike), unusual discharge, to assess pubertal development, nocturnal waking/crying which could be caused by threadworm, fused labia / vulvovaginitis (both v common in pre pubertal girls). And safeguarding where it is necessary to see every part of the body surface (but not a VE unless a sexual assault of some kind was suspected and that is highly specialised and even then may not occur if it would further traumatise etc).
However, 40 years ago times were possibly very different. The reason might have been for one of the above where we would now view it as inappropriate or unhelpful to perform a VE but could have been seen as standard at the time?
Obviously this was an unpleasant / upsetting experience for you as a child being powerless and not even having a clue why it was happening. It doesn't surprise me that it still plays on your mind. We avoid VE in children for good reason. I would see if there are any notes still available that might clarify what exactly was going on medically.
Only other thing I can think of is urinary catheter insertion which might feel somewhat internal or similar to a VE. However, this would either be for an investigation usually in a hospital setting (putting dyes up in the bladder etc some of which could be relevant to abnormal bed wetting), or in an extreme sort of life-and-death type situation. I can't think of a context for urinary catheter insertion at a GP in a well 8 year old.