HELLP syndrome can, in some rare cases, manifest itself postpartum: I've seen only maybe one or two women come through the Foundation that had it manifest solely postpartum, though a LOT of HELLP cases I have seen that happen near term have consequences for looooong after the delivery process. (Unfortunately, since HELLP is relatively rare, it's harder to aggregate cases!)
It may have been that you had a milder variant of HELLP during your pregnancy, that was sort of masked by your more obvious BP issues, and only manifested itself more severely after delivery. Preeclampsia can manifest up to 6 weeks postpartum, so it stands to reason that your HELLP might have lasted that long as well. Of course, having multiples increases the risk factors for a lot of issues, including preeclampsia and HELLP.
Sorry to hear of all that you are still going through involving your liver and gallbladder, as well as what you have been through in the past with your pregnancies.
You should definitely check out the Foundation's page about HELLP http://www.preeclampsia.org/health-information/hellp-syndrome
and share with your OB if you haven't already. And probably a good idea if you do decide to try again and you haven't already, to consider going to a high-risk specialist and detailing your full history to him or her. The Society of Maternal-Fetal Medicine has a MFM locator on their website if you don't know of one locally: https://www.smfm.org/MFM%20Member%20Locator%20Page.cfm