Hi, thanks for using Petco Pet Education Center, formerly Petcoach! Poor Maxi!
There are many reasons for a senior dog to become weak: kidney, liver problems, arthritis, neurological degeneration of the spine, cancer.
Tick infestations, if severe can make any age dog weak by removing too much blood and causing anemia. Also ticks carry infectious diseases like Lyme, Ehrlichia, anaplasma and many others.
I recommend, ideally, to have Maxi seen by a Vet and have blood work done and check her for the above 3 tick borne diseases. Then if healthy enough give her a flea and tick preventative. There are great drugs out now for fleas and ticks, like Nexgard, Simparica, Bravecto and the Seresto collar.
I am so sorry that Maxi is sick and I wish her a speedy recovery! If you need anything else please contact us, we are here to help!