Is there a time to worry about multiple lipomas? History of melanoma.

Thank you for sharing your excellent question.  Because of your melanoma history it is important that you maintain regular follow up appointments with your surgical oncologist to ensure these new lipomas are not in fact lymph nodes that have become enlarged.  As we do not know the full details of your procedure and recovery it is always best to ask you surgeons for advice, and if you are not satisfied, get second opinions.  Best wishes. 

It is not possible to give you a diagnosis over the internet. Multiple lipomas on the forearms is a relatively common condition. Usually the lumps are slow growing or static.
However, if you ask could one of the lumps be a recurrence of melanoma then the answer is yes it is a possibility. One would be more suspicious of new lumps that grew rapidly and particularly if they have bruising associated with them (sometimes melanoma deposits bleed quite easily). If you have a particular lump that you feel is different from your others then clearly you should get it checked.
In the UK you would normally be on 3 monthly check ups with your surgeon/oncologist which is an opportunity to raise your concerns. We would also routinely obtained CT scans typically every 6 months. An alternative which is usually quite available is an ultra sound scan which is a relatively less expensive technique which may be helpful in distinguishing a lipoma from melanoma.
I do not think you can rely on internet diagnosis for your question. I think you should seek the advice from a suitably trained and experienced specialist to give you specific advice relevant to you and your medical history in the light of a medical examination. 
Best wishes