I had microdermabrasion 4 days ago and instantly all these little bumps/rash appeared on my skin -- is this a reaction? (Photo)

The rash you’re experiencing might be a result of harsh abrasion, “overtreating” the area or clogged up pores. The best course of action would be seeing a dermatologist for identifying the cause and getting the right treatment. 

First, let me say that I am sorry you are experiencing this reaction, which quite possibly might be the result of excessive physical abrasion, reaction to the proximate application of certain chemicals on the raw skin following or preceding treatment, or perhaps a combination of these factors. You would do well to consult with a board certified dermatologist for evaluation and treatment. This of course begs the question of the true value of microdermabrasion in the first place and whether it is even worth the risk of any possible complications, such as you might be experiencing now. In the early days, microdermabrasion was called epidermabrasion, which is the kind of superficial skin exfoliation that one might get from an aggressive use of a Buf Puf, Loofah or even a stiff wash cloth.Unfortunately, there is little compelling evidence to suggest that such skin exfoliation has much lasting benefit for treating much of anything, including acne or skin aging. It's OK if one is going for a pampering, spa experience. But if there is some preexistent skin problem for which treatment is being sought, a visit to the dermatologist would likely be more productive and cost effective in the end. So, going forward, you might keep this in mind. Best of luck.

Although microdermabrasion is  very superficial it can uncover some stopped up pores and if too much pressure is applied can look like the skin has been scraped.  see a board certified dermatologist for treatment