Hello !
My female dog (cross-7years old) has bumps underneath her skin all around her hind area (top of tail, top of thighs)& it's itching her a lot. I've just noticed so maybe she's had them a day or two already.
What should I do ?

You should take her to the vet before it turns into something more serious.

Tick

In the summer it's usually either fleas or a heat rash.
Fleas of course go for the head and tail, because they need to drink once a day and those are the moist areas. One application of Frontline or Advantage will kill them in two minutes.
Heat rash will often produce bald spots.
I would take her to a lake for a long swim, or you could give her an oatmeal bath, getting her to sit in it for a while, and pour buckets of rinse water with a little vinegar over the hind area.
Or go to a pet store and show it to them.
Or lastly, if you can afford it - the vet.

Whatever the problem is, really difficult to say without seeing it, if she is really itchy she could traumatize the skin by chewing at the area.
You need to relieve the itching a soothing bath as Joypulv suggested will help with this.
Check her coat for fleas by actually rubbing the fur the wrong way in that area, slowly so that you can get a good look. They tend to go for the warmer moister areas, so roll her over and look at her tummy back between her back legs also.
Sometimes even if you don't see fleas you can tell they have been there by the presence of flea dirt, tiny specks of black stuff, if you see something suspicious, comb some out onto a cloth or paper towel and wet it, flea dirt will turn an orange to red color when wet.
If you see fleas or flea dirt then you will need to treat her and her bedding, basically anywhere that she spends time.
If it is not a flea issue, another possibility is that she has encountered something she is having an allergic reaction to.
Check the spot often, moist and gooey = a possible hot spot, which can lead to a skin infection, because she probably will not leave it alone.
You can safely clean the spots with a little betadine scrub, just be sure to rinse it well afterward.
If you see the area getting worse and not better, you may be looking at a trip to the Vet.
Sorry I got a little long winded, just trying to cover the most possible things.
Good Luck
Keep us posted

I agree with joypulv, fleas would be the most common cause, especially in the areas you are talking about.
When mine have had fleas they always hang around the back end of the dog, especially near the tail base and legs.
They can be hard to see even in short coated dogs so I find the easiest way to tell if they have them is to look and feel for flea dirt. Flea dirt is basically flea poop, it consists mostly of blood and appears as grainy, black dots. If you wet these dots and rub them with a tissue they will turn the tissue red.

Dogs can be allergic to fleas and it can cause a lot of irritation and discomfort. I would get her checked over by a vet so they can recommend what course of action to take, getting rid of the fleas (if that is what she has) can be a lengthy process (you will need to treat the dog, house and yard) but once they are gone the irritation will cease.
Topic treatments I would be wary of if the skin is broken, some dogs can have bad reactions to them, but there are oral medications available which your vet can prescribe.

Other than fleas there is also heat rash as joypulv has said, contact allergies and dermatitis, fungal infections, dietary issues, the list really does go on.

EDIT - Sorry ,did not mean to repeat you Ladysam, you seem to have posted while I was typing!