Your H-1B is only for employment at UMass Amherst, and only for the appointment you held at the time of the application to USCIS.
Additional Activities and Honorarium Payments
Do not engage in activities outside UMass Amherst. Unless you have received USCIS approval for a “concurrent” H-1B through another employer, you may not work elsewhere. Although you are permitted to make occasional speeches and give lectures at other institutions or conferences, you may not receive compensation for these activities. Under the H-1B category, you are not allowed to receive honorarium payments of any sort. However, you may be reimbursed for “transportation and reasonable, incidental expenses” incurred in connection with travel to other institutions or conferences.