Ok. There is some misconceptions here.
In quantum fluctuations, there are being created a pair of virtual particles, one with positive energy/mass, and one with negative energy/mass, because energy cannot be created out of nothing.
This is wrong in many levels.
Quantum fluctuations are not usual fluctuations where things are happening all the time. In quantum mechanics is only useful to talk about what you can measure. Quantum fluctuations are fluctuations that uccurs on the outcomes of a measurement.
Turns out that there is a some sort of counterfactual reasoning, the superposition principle, that help us to calculate things. This is why we call this particles virtual. They are not there,they simple help us to calculate some things. This virtual particles could show's up if you try to measure it, but only in that way.
An important point here is that the process of a absorption of an antiparticle and a emission of a particle is indistinguishable. So, you can always see such a process of pair creation and annihilation as a "propagation" between space-like intervals of a particle only or an antiparticle only, in opposite directions. See here
Energy are not conserved if you measure a pair particle-antiparticl creating from nothing. Actually this process of virtual particles has to do with the fact that energy and momentum fluctuate when you probe small regions of space-time. Besides, each particle has positive energy. The reason why they shows up by pairs of particle and anti-particle is because charge are conserved.
For particles that don't carries any conserved charge they antiparticles are indistinguishable from particles. This means that now the act of absorption or emission of a particle is the same one. And is more intuitive to work with the picture that this particles are propagating between space-like intervals.
