Credit: Steve Granitz/; Splash News Online

Jealous much?

Marc Anthony is up in arms that his ex Jennifer Lopez has moved on since their July split with a much younger man. A source tells the new Us Weekly, on stands Friday, that 43-year-old Anthony is "going nuts" over Lopez's relationship with dancer beau Casper Smart, 24.

"He has fits and tantrums and is very stressed... [He's] lost a lot of weight," a source adds. (A rep for the Latin crooner counters this, telling Us Anthony has "been the same weight most of his life.)

But pals of Lopez, 42, aren't crazy about Smart either. "They don't feel Casper is good enough for her," a source tells Us. "They are worried because she really puts her heart out there for love."

For more on Anthony's jealousy and Lopez's fast track romance with Smart, check out the new Us Weekly, on stands Friday.