Yes Lyme disease has a lot of different ways it expresses. Also Lyme is only one of many TBDs that are worrisome.

Not to be alarmist but- are you saying that all your vet has done is a shot every six months? I only ask because a lot of vets are very uneducated about tick borne diseases and a lot of dogs die because of it. You should most certainly be doing a blood panel every 6 months, but you also should have started with a long course of aggressive antibiotics. Even if your dog does not have symptoms incorrectly treating TBDs can be deadly in the long run. I have never heard of giving a shot every six months to "keep it at bay" but maybe it is a new thing. There are a lot of good resources about tick borne diseases in dogs online- it might be worth some research.