Red, warm, painful lump on the leg

Hi, I know no one will be able to tell for sure what that could be, but it come from nowhere. I'm regularly checking my dog and havent notice this before. He will be 11 years old in August. He have this pink, warm lump on his back leg. Clearly he dont want me touch it for too long. He dont mind me too as he know this is part of our routine. It feel warm when I'm touching it. No changes in eating/ drinking. Active as much as he can be (joint problems). I couldn't notice anything like Mark after bite. Nothing sticking out to be something inside. Quite hard when I touch it. Any closer idea if that could be lipoma? Follicular Cysts? does tumors look like this?

Hello - and I apologize in advance for this frustrating answer, but it is the truth:  it is impossible to tell from the outside of a lump what the lump is.  I can - and will - tell you some things that this one *might* turn out to be, but as the outcome of these varies enormously, it won't be very much use to you.  For example, if it is cancer, it may be best tested or removed or both.  Most cancers are not painful, but some are, for example if they interfere with weight-bearing.  If it may be a benign cancer, your vet may elect to leave it be for a while (some benign tumours, for example a histiocytoma, may regress by themselves after a few weeks).  If it is infection or an abscess, it may need to be lanced;  if there was a bite wound under there, it may need antibiotics.  Demodex mites can stimulate a nodular reaction and are targeted with drugs.  Cysts can vary. And so on.  It falls to your vets' lot to try and do the right thing before they know the answer, although there will be clues on an examination (temperature, level of pain, appearance to a much lesser extent, depth and so on) which will help them to figure out the most logical step.  Sometimes this is obvious, other times two different vets may elect to rule different things out first.  There are a lot of articles about lumps in the blog;  here are two. https://vethelpdirect.com/vetblog/2019/11/04/my-pet-has-a-lump-what-should-i-do/ https://vethelpdirect.com/vetblog/2014/07/24/ask-a-vet-online-my-10-year-old-dog-has-a-lump-not-sure-if-i-should-take-him-to-the-vet/

Lumps connected with allergies are also common on feet, more frequently multiple and between the toes.  In short, it is worth seeing a vet, who will decide what may be most likely (but remember that lumps can be very deceiving) and start to work out an approach.

Here are those links again from the Blog.  The first:  My 10-year old dog has a lump not sure if I should take him to the vet    .   https://vethelpdirect.com/vetblog/2014/07/24/ask-a-vet-online-my-10-year-old-dog-has-a-lump-not-sure-if-i-should-take-him-to-the-vet/

The second:   My pet has a lump  https://vethelpdirect.com/vetblog/2019/11/04/my-pet-has-a-lump-what-should-i-do/

Wishing you both the best of luck