  The TeV observations of GRB~221009A provided us with a unique opportunity to
analyze the contemporaneous phase in which both prompt and afterglow emissions
are seen simultaneously. To describe this initial phase of Gamma-Ray Burst
afterglows, we suggest a model for a blast wave with an intermittent energy
supply. We treat the blast wave as a two-element structure. The central engine
supplies energy to the inner part (shocked ejecta material) via the reverse
shock. As the shocked ejecta material expands, its internal energy is
transferred to the shocked external matter. We take into account the inertia of
the shocked external material so that the pressure difference across this
region determines the derivative of the blast wave's Lorentz factor. Applied to
GRB~221009A, the model yields a very good fit to the observations of the entire
TeV lightcurve except for three regions where there are excesses in the data
with respect to the model. Those are well correlated with the three largest
episodes of the prompt activity, and thus, we interpret them as the reverse
shock emission. Our best-fit solution for GRB~221009A is an extremely narrow
jet with an opening angle theta_j approx 0.07^o (500/\Gamma_0) propagating into
a wind-like external medium. This extremely narrow angle is consistent with the
huge isotropic equivalent energy of this burst, and its inverse jet break
explains the very rapid rise of the afterglow. Interestingly, photon-photon
annihilation doesn't play a decisive role in the best-fit model.
