Unfortunately you may not have a say in this. Your insurance may put you on Venlafaxine, because it's cheaper. If you plan to be on antidepressant for a long time (and you should be, if you have a history of panic attacks), your insurance would like to cut costs over the long run, with the cheaper priced generic. Not to mention, Effexor and the other SNRIs, are probably the most expensive antidepressants on the market, often ranging around $300 for monthly prescriptions.
If you've been on Effexor before over several years, and then stopped, often a user will have to take a higher dose to get the same effect as the first time. This is one of the side effects of start and stop antidepressant usage, which many antidepressant users don't get informed about, when they first stopped their medication.
Since you've currently been on Venlafaxine for only two months, your brain is still adjusting to the medication. It takes about three months to expect a stable medication dosage. To counter act the increased anxiety, you should meet with your psychiatrist about possibly upping the Venlafaxine to 100mg.
Aside from that, I don't think the generic makes the medication less potent. This 'generic vs original' issue has been raised by several posters on Anxiety Zone, and the same response is "No, there is not a big difference between the generic and the original." Some have even claimed a different color of the same medication could be the result of increased anxiety, but that argument doesn't fare well either. I take the generic Setraline for Zoloft, and my anxiety disorder is under control.
* Please reference this post for more information on how a second time antidepressant use may loose effectivity, and require greater dosage than before.http://www.ncbi.nlm.nih.gov/pubmed/18694599