I hope I can help you with this. Online of course, no one can say with any certainty what anything is, all we can do is offer our best guesses based on what is written.
Its interesting that you say this rash started with your pregnancy, as there are specific rashes that occur in pregnancy, and most of the time, they go away shortly after the baby is born. Such rashes include pruritic urticarial papules and plaques of pregnancy (PUPP), pemphigoid gestationis (which was once called herpes gestationis but as it had nothing do to with the herpes virus the name was changed), impetigo herpetiformis, papular dermatitis of pregnancy, and prurigo gestationis.
Reading about these, it may be possible to eliminate some based on the characteristics of the rash. Of course, I cant state its NOT these, but it may be less likely. For instance, impetigo herpetiformis is typically accompanied by a fever and feelings of illness and malaise, and PUPP resolves after pregnancy and does not usually recur. And, PUPP is known for appearing in stretch marks/ striae. Papular dermatitis of pregnancy is an itchy rash that looks almost like insect bites, and some of the areas may even be crusted over.
It may be possible that your rash might be pemphigoid gestationis. It is associated with fluid filled blisters, and may recur from, time to time after the baby is delivered. It is thought to be related to an auto-immune process; thats when the body's white cells attack the body mistakenly, believing it to be a pathogen. However, the blisters of this condition tend to be rather large affecting wide areas; your description is small blisters, and affecting about an inch in area. Treatment is more symptomatic here; aimed at relieving the discomfort. Mild cases can be treated with hydrocortisone creams and antihistamines. Here is more information on this condition: There are three pictures of this in this article; of severe cases; the second one of the rash behind the ear may relate to you. http://emedicine.medscape.com/article/1063499-overview
I asked about chicken pox as shingles was a brief thought of mine, but shingles tends to appear in dermatones, along the path that nerves follow, and your presentation doesn't quite match. Shingles can also take weeks to resolve, and can be associated with intense pain as well as itching.
If you've not consulted a dermatologist, it may be wise to so; even if you don't have the rash at the time of the visit, you can tell him the story behind it, and show him pictures of the rash. A dermatologist may be more familiar with rashes of pregnancy than a general practitioner.
I hope I was able to help, and thank you for this interesting question. I hope you can get a diagnosis soon; it certainly is annoying to have recurrent rashes.
Here is more information regarding pregnancy rashes in general: http://www.netdoctor.co.uk/womenshealth/facts/skinrash_pregnancy.htm