Biting tail really hard?

Luna always chases her tail, which I just assumed is a normal puppy thing to do, but yesterday she bit it so hard that it has a small cut on it now. Nothing huge or anything just tiny bleeding cut. My worry is that she’s really hurting herself and don’t really understand why? I have asked my vet to check her since I read that it could be over parasites etc but she is wormed and vaccinated and is healthy so I am ruling that out.its just when she gets playful and sees her tail with corner of her eye she attacks it but is very rough with it like she wants it gone from her...?

Use distraction techniques and reward for good behaviour

It's basically OCD. Or, CCD (Canine Compulsive Disorder). It isn't her fault, it isn't your fault - it's a condition that affects hundreds of dogs! 

Every time she chases her tail you must stop her - use treats, toys, whatever it rakes to interrupt the behaviour. 

So many people find OCD in dogs funny, but it's no laughing matter & can lead to some nasty injuries.  You need to step on it NOW to stop further problems.

Some myths say it’s stress related but definitely get those squeaky toys out to distract her

As said it could be a sign of CCD. When she gets a little too hyper unless she has something else to focus on she will invert and focus on herself..so the main thing you can do is limit how excited you make her for now and make all play time focused on training and calming traits like "wait" and stay etc and reward those with a calm positive reaction. Eg do the act, then oh good girl and then ignore for a bit, so she doesn't get hyped up.

I see, thank you for the information. It always starts out when she’s hyper and plays with her toys for a bit when she starts doing it she’s full on pulling at her tail with her front teeth which is why I started getting worried as her bites hurt and her tail is so tiny thing is when she starts biting it she doesn’t care about toys at all, takes a good few minutes of literally dragging her head away and putting a toy in her mouth to get her to stop.. she gets hyper so easily though it’s hard not to make her excited but she’s starting to respond more to me trying to calm her so hopefully comes more in time

That’s one thing that can be difficult with a puppy, you deciding and enforcing where the limits are. Maybe just try to lower the pressure before she gets too wound up and this tail chasing will soon be a memory from the past

Can I ask what food she's getting?

I was literally just about to say what is she fed too..reason being a lot of foods can have sugars and additives and despite being "good brands" will be basically like candy for a dog. So many behaviours can be helped with a more appropriate diet.

I get royal canning for medium dog puppies, it’s the food the breeder used but the vets here all recommended using royal canning as well so we’re trying it out for a while, she seems to love it anyway. Sometimes she also gets “treat food” like boiled rice with carrots and chicken mixed with the nuts or a bit of boiled egg in her nuts. Not often though

The grain in the food could be the problem perhaps grain free could help iff you do decide to change your dogs food do it slowly over a period of a few weeks gradually introduce the new food and eliminating the old food it will reduce stomach upset. Ps: a raw egg is better than a boiled one

Royal Canin isn't a terrible food, but isn't good, either! Vets do recommend it because it's a tested food for general ailments.

Have a look at the allaboutdogfood site & try to choose a 4 or 5 star food.

There is a reason vets have a feed stand in the waiting area which is usually royal canin or science plan...and it is not because they truly believe it is the best ever food any dog can eat. Don't go by what a breeder has fed or what a vet says is ok neither are nutritionists.