You do need to see an ophthalmologist for a complete exam and some special tests. the most common cause of this would be your contact lens warping your cornea. That can take several months to correct. Other causes would include other cornea diseases such as keratoconus, cataract formation, macular problems. You should see a Eye MD ophthalmologist, if possible one that specializes in corneal disease.
No it doesn't. I can't tell you what the problem is. As I said if you are young and healthy the most common cause would be corneal warping due to your contacts.
Dry eyes might be a possible explanation since they are a common cause of halos, I would expect your doctor to consider that possibility.
The fact that you had a "slit lamp" examination doesn't tell us if you went to an optometrist or an ophthalmologist. Both are general eye doctors (at least in the US, this is a global site so I don't know if you are here, in some countries optometrists don't have as much background), but an ophthalmologist has a general medical degree and tends to have a wider range of medical eye conditions they can deal with. They also are the only ones here that can perform cataract or other surgeries so they are often more familiar with diagnosing issues that can only be dealt with surgically.
Although it isn't the most likely explanation if you are young (the fact that you've only worn contacts for 2 years suggests the possibility you are young, though people might switch to them at any age), it is possible for an optometrist to miss a cataract at the slit lamp, especially if you are young enough that cataracts aren't common. My optometrist had seen a major decrease in visual acuity in one eye over a few months, and a major change in astigmatism, and couldn't diagnose the problem. After exploring possibilities like stoping contact lens wear, the doc finally gave up and referred me to an ophthalmologist who diagnosed the issue as a cataract. The OD had only noted "trace nuclear sclerotic changes" in my eye's natural lens but hadn't considered it enough to be labeled a cataract or to be causing the trouble, perhaps partly because at even at 49 I was young to have a problem cataract. As it became worse the cataract was more obviously visible to the optometrist.
Ophthalmologists also usually have more of a complete suite of scanning equipment like equipment that will map the shape of your cornea. (I have seen some optometrists advertising scanners, but usually not the full suite that a good ophthalmologists office will have, though you may need to hunt to be sure you have a good one).
Again no way to tell you for sure but if you see it every time, its on the lower part of the eye (not the eyelid) and it moves when you blink most likely you are seeing movement of the bulbar conjunctiva. this is the layer of tissue over the eyeball (sclera).