Rosemary prefers poor-quality soil, so frequent, heavy fertilizing is incorrect. The rosemary doesn't like it. In its natural habitat--the northern Mediterranean coast--rosemary grows in sandy soil, in windswept areas, and is beaten down (we would think, at least) by the sun several months out of the year. It receives water during the winter rains--the rainy season there--and rarely during the months from April through October/early November--the dry season. March may or may not provide rain. (The San Francisco Bay Area has a Mediterranean climate.)
You're probably guilty of nothing more than treating the rosemary too
In case anyone thinks this is an exaggeration, here's what the Sunset Western Garden Book
(2001 ed.) says:
The genus name means 'dew of the sea,' reflecting the plant's native habitat on seaside cliffs in the Mediterranean region. Tough and versatile, rosemary grows most luxuriantly just above the tide line, braving wind and salt spray--but it will thrive inland, even enduring blistering sun and poor alkaline soil, if given moderate water and infrequent light feeding." (p. 587)
Let the plant go a little drier than you're comfortable with, and don't fertilize it for the next eight to ten weeks. Take your cues for watering and *light* fertilization from the plant itself: insert your fingers into the growing medium (potting soil). If your fingers are dry to the second knuckle (approx. 4.5 cm on my right index finger; I'm a relatively short woman), then water the plant. If the soil is still wet, don't water yet.
When the leaves are green again (new growth is unaffected and all green), perhaps a 25% strength feeding of fertilizer will be helpful. But, honestly, my rosemary is in the ground and I have never
fertilized it in the 15, almost 16
years we've lived in this house. Container plants do need some reinforcements, though, which is why I recommend the 25% strength fertilizer, based on Sunset's "light feeding" statement.
Give it as much sun as you can.
Sunset Zone 17, USDA Zone 9